v3.25.4

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2025	Jan. 31, 2026	Jun. 30, 2025
Entity Addresses [Line Items]
Document Annual Report	true
Document Period End Date	Dec. 31, 2025
Document Transition Report	false
Entity File Number	1-2328
Entity Incorporation, State or Country Code	NY
Entity Tax Identification Number	36-1124040
Entity Address, Address Line One	233 South Wacker Drive
Entity Address, City or Town	Chicago
Entity Address, State or Province	IL
Entity Address, Postal Zip Code	60606
City Area Code	(312)
Local Phone Number	621-6200
Entity Interactive Data Current	Yes
Document Fiscal Year Focus	2025
Document Fiscal Period Focus	FY
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Documents Incorporated by Reference	GATX’s definitive Proxy Statement to be filed on or about March 13, 2026
ICFR Auditor Attestation Flag	true
Amendment Flag	false
Entity Central Index Key	0000040211
Entity Common Stock, Shares Outstanding		35,500,000
Entity Well-known Seasoned Issuer	Yes
Entity Public Float			$ 5,400.0
Document Type	10-K
Entity Registrant Name	GATX Corporation
Current Fiscal Year End Date	--12-31
Document Financial Statement Error Correction [Flag]	false
NEW YORK STOCK EXCHANGE, INC. [Member]
Entity Addresses [Line Items]
Security Exchange Name	NYSE
Title of 12(b) Security	Common Stock
Trading Symbol	GATX
NYSE Texas, Inc [Member]
Entity Addresses [Line Items]
Security Exchange Name	CHX
Title of 12(b) Security	Common Stock
Trading Symbol	GATX

Boolean flag that is true when the XBRL content amends previously-filed or accepted submission.

+ References

No definition available.

+ Details

Name:	dei_AmendmentFlag
Namespace Prefix:	dei_
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	duration

- Definition

Area code of city

+ References

No definition available.

+ Details

Name:	dei_CityAreaCode
Namespace Prefix:	dei_
Data Type:	xbrli:normalizedStringItemType
Balance Type:	na
Period Type:	duration

- Definition

End date of current fiscal year in the format --MM-DD.

+ References

No definition available.

+ Details

Name:	dei_CurrentFiscalYearEndDate
Namespace Prefix:	dei_
Data Type:	xbrli:gMonthDayItemType
Balance Type:	na
Period Type:	duration

- Definition

Boolean flag that is true only for a form used as an annual report.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Form 10-K
-Number 249
-Section 310

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Form 20-F
-Number 249
-Section 220
-Subsection f

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Form 40-F
-Number 249
-Section 240
-Subsection f

+ Details

Name:	dei_DocumentAnnualReport
Namespace Prefix:	dei_
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	duration

- Definition

Indicates whether any of the financial statement period in the filing include a restatement due to error correction.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-K
-Number 229
-Section 402
-Subsection w

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Form 10-K
-Number 249
-Section 310

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Form 20-F
-Number 249
-Section 220
-Subsection f

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Form 40-F
-Number 249
-Section 240
-Subsection f

+ Details

Name:	dei_DocumentFinStmtErrorCorrectionFlag
Namespace Prefix:	dei_
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	duration

- Definition

Fiscal period values are FY, Q1, Q2, and Q3. 1st, 2nd and 3rd quarter 10-Q or 10-QT statements have value Q1, Q2, and Q3 respectively, with 10-K, 10-KT or other fiscal year statements having FY.

+ References

No definition available.

+ Details

Name:	dei_DocumentFiscalPeriodFocus
Namespace Prefix:	dei_
Data Type:	dei:fiscalPeriodItemType
Balance Type:	na
Period Type:	duration

- Definition

This is focus fiscal year of the document report in YYYY format. For a 2006 annual report, which may also provide financial information from prior periods, fiscal 2006 should be given as the fiscal year focus. Example: 2006.

+ References

No definition available.

+ Details

Name:	dei_DocumentFiscalYearFocus
Namespace Prefix:	dei_
Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	duration

- Definition

For the EDGAR submission types of Form 8-K: the date of the report, the date of the earliest event reported; for the EDGAR submission types of Form N-1A: the filing date; for all other submission types: the end of the reporting or transition period. The format of the date is YYYY-MM-DD.

+ References

No definition available.

+ Details

Name:	dei_DocumentPeriodEndDate
Namespace Prefix:	dei_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Boolean flag that is true only for a form used as a transition report.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Forms 10-K, 10-Q, 20-F
-Number 240
-Section 13
-Subsection a-1

+ Details

Name:	dei_DocumentTransitionReport
Namespace Prefix:	dei_
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	duration

- Definition

The type of document being provided (such as 10-K, 10-Q, 485BPOS, etc). The document type is limited to the same value as the supporting SEC submission type, or the word 'Other'.

+ References

No definition available.

+ Details

Name:	dei_DocumentType
Namespace Prefix:	dei_
Data Type:	dei:submissionTypeItemType
Balance Type:	na
Period Type:	duration

- Definition

Documents incorporated by reference.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Exchange Act
-Number 240
-Section 12
-Subsection b-23

+ Details

Name:	dei_DocumentsIncorporatedByReferenceTextBlock
Namespace Prefix:	dei_
Data Type:	dtr-types:textBlockItemType
Balance Type:	na
Period Type:	duration

- Definition

Address Line 1 such as Attn, Building Name, Street Name

+ References

No definition available.

+ Details

Name:	dei_EntityAddressAddressLine1
Namespace Prefix:	dei_
Data Type:	xbrli:normalizedStringItemType
Balance Type:	na
Period Type:	duration

- Definition

Name of the City or Town

+ References

No definition available.

+ Details

Name:	dei_EntityAddressCityOrTown
Namespace Prefix:	dei_
Data Type:	xbrli:normalizedStringItemType
Balance Type:	na
Period Type:	duration

- Definition

Code for the postal or zip code

+ References

No definition available.

+ Details

Name:	dei_EntityAddressPostalZipCode
Namespace Prefix:	dei_
Data Type:	xbrli:normalizedStringItemType
Balance Type:	na
Period Type:	duration

- Definition

Name of the state or province.

+ References

No definition available.

+ Details

Name:	dei_EntityAddressStateOrProvince
Namespace Prefix:	dei_
Data Type:	dei:stateOrProvinceItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	dei_EntityAddressesLineItems
Namespace Prefix:	dei_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

A unique 10-digit SEC-issued value to identify entities that have filed disclosures with the SEC. It is commonly abbreviated as CIK.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Exchange Act
-Number 240
-Section 12
-Subsection b-2

+ Details

Name:	dei_EntityCentralIndexKey
Namespace Prefix:	dei_
Data Type:	dei:centralIndexKeyItemType
Balance Type:	na
Period Type:	duration

- Definition

Indicate number of shares or other units outstanding of each of registrant's classes of capital or common stock or other ownership interests, if and as stated on cover of related periodic report. Where multiple classes or units exist define each class/interest by adding class of stock items such as Common Class A [Member], Common Class B [Member] or Partnership Interest [Member] onto the Instrument [Domain] of the Entity Listings, Instrument.

+ References

No definition available.

+ Details

Name:	dei_EntityCommonStockSharesOutstanding
Namespace Prefix:	dei_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Indicate 'Yes' or 'No' whether registrants (1) have filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that registrants were required to file such reports), and (2) have been subject to such filing requirements for the past 90 days. This information should be based on the registrant's current or most recent filing containing the related disclosure.

+ References

No definition available.

+ Details

Name:	dei_EntityCurrentReportingStatus
Namespace Prefix:	dei_
Data Type:	dei:yesNoItemType
Balance Type:	na
Period Type:	duration

- Definition

Indicate if registrant meets the emerging growth company criteria.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Exchange Act
-Number 240
-Section 12
-Subsection b-2

+ Details

Name:	dei_EntityEmergingGrowthCompany
Namespace Prefix:	dei_
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	duration

- Definition

Commission file number. The field allows up to 17 characters. The prefix may contain 1-3 digits, the sequence number may contain 1-8 digits, the optional suffix may contain 1-4 characters, and the fields are separated with a hyphen.

+ References

No definition available.

+ Details

Name:	dei_EntityFileNumber
Namespace Prefix:	dei_
Data Type:	dei:fileNumberItemType
Balance Type:	na
Period Type:	duration

- Definition

Indicate whether the registrant is one of the following: Large Accelerated Filer, Accelerated Filer, Non-accelerated Filer. Definitions of these categories are stated in Rule 12b-2 of the Exchange Act. This information should be based on the registrant's current or most recent filing containing the related disclosure.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Exchange Act
-Number 240
-Section 12
-Subsection b-2

+ Details

Name:	dei_EntityFilerCategory
Namespace Prefix:	dei_
Data Type:	dei:filerCategoryItemType
Balance Type:	na
Period Type:	duration

- Definition

Two-character EDGAR code representing the state or country of incorporation.

+ References

No definition available.

+ Details

Name:	dei_EntityIncorporationStateCountryCode
Namespace Prefix:	dei_
Data Type:	dei:edgarStateCountryItemType
Balance Type:	na
Period Type:	duration

- Definition

Boolean flag that is true when the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S-T during the preceding 12 months (or for such shorter period that the registrant was required to submit such files).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-T
-Number 232
-Section 405

+ Details

Name:	dei_EntityInteractiveDataCurrent
Namespace Prefix:	dei_
Data Type:	dei:yesNoItemType
Balance Type:	na
Period Type:	duration

- Definition

The aggregate market value of the voting and non-voting common equity held by non-affiliates computed by reference to the price at which the common equity was last sold, or the average bid and asked price of such common equity, as of the last business day of the registrant's most recently completed second fiscal quarter.

+ References

No definition available.

+ Details

Name:	dei_EntityPublicFloat
Namespace Prefix:	dei_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The exact name of the entity filing the report as specified in its charter, which is required by forms filed with the SEC.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Exchange Act
-Number 240
-Section 12
-Subsection b-2

+ Details

Name:	dei_EntityRegistrantName
Namespace Prefix:	dei_
Data Type:	xbrli:normalizedStringItemType
Balance Type:	na
Period Type:	duration

- Definition

Boolean flag that is true when the registrant is a shell company as defined in Rule 12b-2 of the Exchange Act.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Exchange Act
-Number 240
-Section 12
-Subsection b-2

+ Details

Name:	dei_EntityShellCompany
Namespace Prefix:	dei_
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	duration

- Definition

Indicates that the company is a Smaller Reporting Company (SRC).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Exchange Act
-Number 240
-Section 12
-Subsection b-2

+ Details

Name:	dei_EntitySmallBusiness
Namespace Prefix:	dei_
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	duration

- Definition

The Tax Identification Number (TIN), also known as an Employer Identification Number (EIN), is a unique 9-digit value assigned by the IRS.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Exchange Act
-Number 240
-Section 12
-Subsection b-2

+ Details

Name:	dei_EntityTaxIdentificationNumber
Namespace Prefix:	dei_
Data Type:	dei:employerIdItemType
Balance Type:	na
Period Type:	duration

- Definition

Indicate 'Yes' or 'No' if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Act.

+ References

No definition available.

+ Details

Name:	dei_EntityVoluntaryFilers
Namespace Prefix:	dei_
Data Type:	dei:yesNoItemType
Balance Type:	na
Period Type:	duration

- Definition

Indicate 'Yes' or 'No' if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act. Is used on Form Type: 10-K, 10-Q, 8-K, 20-F, 6-K, 10-K/A, 10-Q/A, 20-F/A, 6-K/A, N-CSR, N-Q, N-1A.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Securities Act
-Number 230
-Section 405

+ Details

Name:	dei_EntityWellKnownSeasonedIssuer
Namespace Prefix:	dei_
Data Type:	dei:yesNoItemType
Balance Type:	na
Period Type:	duration

- References

+ Details

Name:	dei_IcfrAuditorAttestationFlag
Namespace Prefix:	dei_
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	duration

- Definition

Local phone number for entity.

+ References

No definition available.

+ Details

Name:	dei_LocalPhoneNumber
Namespace Prefix:	dei_
Data Type:	xbrli:normalizedStringItemType
Balance Type:	na
Period Type:	duration

- Definition

Title of a 12(b) registered security.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Exchange Act
-Number 240
-Section 12
-Subsection b

+ Details

Name:	dei_Security12bTitle
Namespace Prefix:	dei_
Data Type:	dei:securityTitleItemType
Balance Type:	na
Period Type:	duration

- Definition

Name of the Exchange on which a security is registered.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Exchange Act
-Number 240
-Section 12
-Subsection d1-1

+ Details

Name:	dei_SecurityExchangeName
Namespace Prefix:	dei_
Data Type:	dei:edgarExchangeCodeItemType
Balance Type:	na
Period Type:	duration

- Definition

Trading symbol of an instrument as listed on an exchange.

+ References

No definition available.

+ Details

Name:	dei_TradingSymbol
Namespace Prefix:	dei_
Data Type:	dei:tradingSymbolItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	dei_EntityListingsExchangeAxis=exch_XNYS
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_EntityListingsExchangeAxis=gmt_NYSETexasIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

GATX CORP 10-K