Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2022	Apr. 30, 2022
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2022
Document Fiscal Year Focus	2022
Document Fiscal Period Focus	Q1
Trading Symbol	RNDB
Entity Registrant Name	Randolph Bancorp, Inc.
Entity Central Index Key	0001667161
Current Fiscal Year End Date	--12-31
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, 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 Quarterly Report	true
Document Transition Report	false