Global Pandemic Affecting Randolph Bancorp, Inc. - Summary of Loan Portfolio (Detail) - USD ($) $ in Thousands		Mar. 31, 2021	Dec. 31, 2020
Financing Receivable Recorded Investment Past Due [Line Items]
Deferrals Granted	[1]	$ 58,574	$ 57,510
Suspended/Reduction Payment	[2]	11,018	20,562
Resumed Payment	[2]	37,163	30,437
Full Payoff	[3]	10,393	6,511
Residential Real Estate [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Deferrals Granted	[1]	54,927	53,797
Suspended/Reduction Payment	[2]	10,980	20,494
Resumed Payment	[2]	35,564	28,802
Full Payoff	[3]	8,383	4,501
Residential Real Estate [Member] | One-to-Four Family [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Deferrals Granted	[1]	16,793	15,514
Suspended/Reduction Payment	[2]	4,316	12,974
Resumed Payment	[2]	9,107	2,109
Full Payoff	[3]	3,370	431
Residential Real Estate [Member] | Home Equity Loans and Lines of Credit [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Deferrals Granted	[1]	3,168	3,179
Suspended/Reduction Payment	[2]	423	626
Resumed Payment	[2]	1,479	1,287
Full Payoff	[3]	1,266	1,266
Residential Real Estate [Member] | Construction [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Deferrals Granted	[1]	350	350
Suspended/Reduction Payment	[2]		350
Resumed Payment	[2]	350
Residential Real Estate [Member] | Commercial Real Estate Loans [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Deferrals Granted	[1]	34,616	34,754
Suspended/Reduction Payment	[2]	6,241	6,544
Resumed Payment	[2]	24,628	25,406
Full Payoff	[3]	3,747	2,804
Commercial and Industrial [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Deferrals Granted	[1]	3,579	3,645
Resumed Payment	[2]	1,569	1,635
Full Payoff	[3]	2,010	2,010
Consumer [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Deferrals Granted	[1]	68	68
Suspended/Reduction Payment	[2]	38	$ 68
Resumed Payment	[2]	$ 30

[1] This column equals the current outstanding balance of loans that received a deferral, plus the balance of loans that were deferred and were paid off in full.

[2] These two columns are the current balance of all loans that received a deferral. The Suspended/Reduced Payment columns represents loans currently in a deferral period and the Resumed Payment columns represents loans that are no longer in a deferral period and have resumed normal payment.

[3] This column represents the balance of deferred loans that were paid off in full.