Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2021	Apr. 15, 2022	Jun. 30, 2021
Cover [Abstract]
Document Type	10-K/A
Amendment Flag	true
Amendment Description	Randolph Bancorp, Inc. (the “Company”) is filing this Amendment No. 1 (this “Amendment”) to its Annual Report on Form 10-K for the fiscal year ended December 31, 2021 (the “Original 10-K”) to provide additional information required by Part III. This Amendment does not change the previously reported financial statements or any other disclosure contained in Part I or Part II in the Original 10-K. Part IV is being amended solely to add new certifications in accordance with Rule 13a-14(a) of the Securities Exchange Act of 1934, as amended.
Document Period End Date	Dec. 31, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Trading Symbol	RNDB
Entity Registrant Name	Randolph Bancorp, Inc.
Entity Central Index Key	0001667161
Current Fiscal Year End Date	--12-31
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	No
Entity Filer Category	Non-accelerated Filer
Entity Current Reporting Status	Yes
Entity Small Business	true
Entity Emerging Growth Company	false
Security Exchange Name	NASDAQ
Title of 12(b) Security	Common Stock, par value $.01 per share
Entity Incorporation, State or Country Code	MA
Entity Interactive Data Current	Yes
Entity Common Stock, Shares Outstanding		5,181,977
Entity Shell Company	false
Entity File Number	001-37780
Entity Tax Identification Number	81-1844402
Entity Address, Address Line One	2 Batterymarch Park
Entity Address, Address Line Two	Suite 301
Entity Address, City or Town	Quincy
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02169
City Area Code	781
Local Phone Number	963-2100
Document Annual Report	true
Document Transition Report	false
Entity Public Float			$ 92,148,669
ICFR Auditor Attestation Flag	false
Auditor Firm ID	173
Auditor Name	Crowe LLP
Auditor Location	New York, New York
Documents Incorporated by Reference	DOCUMENTS INCORPORATED BY REFERENCE None.

Randolph Bancorp, Inc. 10-K/A