Cover Page - USD ($)	12 Months Ended
Cover Page - USD ($)	Dec. 31, 2021	Apr. 22, 2022	Jun. 30, 2021
Document Information [Line Items]
Document Type	10-K/A
Amendment Flag	true
Document Period End Date	Dec. 31, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Document Annual Report	true
Document Transition Report	false
Entity Interactive Data Current	Yes
Entity Registrant Name	Checkmate Pharmaceuticals, Inc.
Entity Central Index Key	0001651431
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Shell Company	false
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Incorporation, State or Country Code	DE
Entity Address, State or Province	MA
Title of 12(b) Security	Common Stock
Trading Symbol	CMPI
Security Exchange Name	NASDAQ
Entity Public Float			$ 124,028,535
Entity Common Stock, Shares Outstanding		22,020,131
ICFR Auditor Attestation Flag	false
Entity File Number	001-39425
Entity Address, Address Line One	245 Main Street
Entity Address, Address Line Two	2nd Floor
Entity Address, City or Town	Cambridge
Entity Address, Postal Zip Code	02142
Entity Tax Identification Number	36-4813934
City Area Code	617
Local Phone Number	682-3625
Auditor Name	KPMG LLP
Auditor Firm ID	185
Auditor Location	Boston, Massachusetts
Amendment Description	EXPLANATORY NOTE Checkmate Pharmaceuticals, Inc. (“Checkmate,” the “Company,” “we,” “us,” and “our”) is filing this Amendment No. 1 on Form 10-K/A for the year ended December 31, 2021 (“Form 10-K/A”) to amend our Annual Report on Form 10-K for the year ended December 31, 2021, originally filed with the Securities and Exchange Commission (the “SEC”) on March 30, 2022 (the “Original Form 10-K”). We are filing this Form 10-K/A to present the information required by Part III of Form 10-K that was previously omitted from the Original Form 10-K in reliance on General Instruction G(3) to Form 10-K, which permits the information in the below referenced items to be incorporated in the Form 10-K by reference from our definitive proxy statement if such statement is filed no later than 120 days after our fiscal year end. We are filing this Form 10-K/A to provide information required in Part III of Form 10-K for the fiscal year ended December 31, 2021, because a definitive proxy statement containing such information will not be filed by us within 120 days after the end of the fiscal year. The Company is hereby amending the Original Form 10-K as follows: • On the cover page, (i) to delete the reference in the Original Form 10-K to the incorporation by reference of the Company’s definitive proxy statement for its 2022 annual meeting of stockholders (the “Annual Meeting”) and (ii) to update the date as of which the number of outstanding shares of the Company’s common stock, par value $0.0001 per share (the “common stock”), is being provided; • To present the information required by Part III of Form 10-K, which information we originally indicated in the Original Form 10-K would be incorporated by reference in our definitive proxy statement to be delivered to our stockholders in connection with our Annual Meeting; and • To amend and restate Part IV, Item 15(b) (Exhibits 31.1 and 31.2) in their entirety to include the currently dated certifications from the Company’s principal executive officer and principal financial officer pursuant to Section 302 of the Sarbanes-Oxley Act of 2002. The certifications of the Company’s principal executive officer and principal financial officer are attached to this Form 10-K/A as Exhibits 31.3 and 31.4. Because no financial statements have been included in this Form 10-K/A and this Form 10-K/A does not contain or amend any disclosure with respect to Items 307 and 308 of Regulation S-K, paragraphs 3, 4, and 5 of the certifications have been omitted. The Exhibit Index has also been amended and restated in its entirety to include the certifications as exhibits. Except as described above, no other changes have been made to the Original Form 10-K. This Form 10-K/A does not otherwise update information in the Original Form 10-K to reflect facts or events occurring subsequent to the filing date of the Original Form 10-K. This Form 10-K/A should be read in conjunction with the Original Form 10-K and with any of our filings made with the SEC subsequent to filing of the Original Form 10-K. Proposed Merger and Expected Form 15 Filing On April 18, 2021, the Company entered into an Agreement and Plan of Merger (the “Merger Agreement”) with Regeneron Pharmaceuticals, Inc., a New York corporation (“Parent”), and Scandinavian Acquisition Sub, Inc., a Delaware corporation and a wholly owned subsidiary of Parent (“Purchaser”). Upon consummation of the Merger, Purchaser will merge with and into the Company, with the Company continuing as a wholly owned subsidiary of Parent (the “Merger”). Pursuant to the Merger Agreement, each issued and outstanding share of common stock (other than (i) shares of common stock with respect to which the holders thereof have properly exercised and perfected demands for appraisal of such shares in accordance with the Delaware General Corporation Law, (ii) shares of common stock that are owned by the Company as treasury stock, and (iii) shares of common stock then held by Parent or Purchaser) will be converted automatically into and will thereafter represent only the right to receive $10.50 in cash, without interest. Immediately prior to the effective time of the Merger (the “Effective Time”), by virtue of the Merger and without any action on the part of any holder thereof, each outstanding and unexercised Company option, whether or not vested, that has a per share exercise price that is less than the Merger Consideration (as defined in the Merger Agreement) shall be cancelled and the holder thereof shall be entitled to receive a cash payment equal to (A) the excess, if any, of (x) the Merger Consideration over (y) the exercise price payable per share under such option, multiplied by (B) the total number of shares subject to such option immediately prior to the Effective Time (without regard to vesting). As a result of the Merger, if consummated, the Company intends to file a Form 15 with respect to all of its registered classes of securities to terminate the registration of such securities under Section 12(g) of the Securities Exchange Act of 1934, as amended, (the “Exchange Act”), and suspend the duty of the Company to file reports under Sections 13 and 15(d) of the Exchange Act. As a result of such termination and suspension, the Company will no longer be required to file reports under Sections 13 and 15(d) of the Exchange Act. Prior to the filing of such Form 15, SEC rules and regulations require the Company to first file this Form 10-K/A in order to complete the Company’s Original Form 10-K and be current in its Exchange Act reporting obligations.

Checkmate Pharmaceuticals, Inc. 10-K/A