Global Pandemic Affecting Randolph Bancorp, Inc. - Summary of Lending Activity and Outstanding Balances of PPP Loans and The Associated Pledges (Detail) $ in Thousands	Mar. 31, 2021 USD ($)
Financing Receivable Recorded Investment Past Due [Line Items]
PPP Loans Originated	$ 25,547
PPP Loan Balance Outstanding	14,717
Amount Pledged To PPLF	15,387
2020 PPP Loans [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
PPP Loans Originated	15,387
PPP Loan Balance Outstanding	4,557
Amount Pledged To PPLF	15,387
2021 PPP Loans [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
PPP Loans Originated	10,160
PPP Loan Balance Outstanding	$ 10,160