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Document and Entity Information - USD ($)
 12 Months Ended
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    
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