Parent Company Condensed Financial Statements - Statements of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015
Condensed Income Statements Captions [Line Items]
Total income	$ 3,923	$ 3,876	$ 3,260	$ 3,155	$ 3,174	$ 3,265	$ 3,072	$ 2,971	$ 14,215	$ 12,482
Charitable foundation contribution									2,275
Income tax expense (benefit)	10			3	(103)	(1)	(1)	(3)	13	(108)
Net income (loss)	$ (546)	$ 289	$ 718	$ 2	$ 259	$ (361)	$ (368)	$ (284)	463	(754)
Randolph Bancorp, Inc. and Predecessor Randolph Bancorp [Member]
Condensed Income Statements Captions [Line Items]
Total income									82
Charitable foundation contribution									2,275
Operating expenses									141
Loss before incomes taxes and equity in undistributed net income (loss) of Randolph Savings Bank									(2,334)
Loss before equity in net income (loss) of Randolph Savings Bank									(2,334)
Equity in undistributed net income (loss) of Randolph Savings Bank									2,797	(754)
Net income (loss)									$ 463	$ (754)