Parent Company Condensed Financial Statements - Statements of Cash Flows (Detail) - USD ($)	1 Months Ended	3 Months Ended								12 Months Ended		24 Months Ended
	Jul. 31, 2016	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	Dec. 31, 2016
Cash flows from operating activities:
Net income (loss)		$ (546,000)	$ 289,000	$ 718,000	$ 2,000	$ 259,000	$ (361,000)	$ (368,000)	$ (284,000)	$ 463,000	$ (754,000)
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Charitable foundation contribution of Company stock										1,820,000
Net cash provided by (used in) operating activities										914,000	(2,715,000)
Cash flows from investing activities:
Acquisition of First Eastern, net of cash acquired										(11,131,000)
Net cash used in investing activities										(31,191,000)	(23,196,000)
Cash flows from financing activities:
Proceeds from issuance of common stock	$ 56,867,500									56,868,000
Stock offering costs										(2,004,000)	(321,000)	$ (2,325,000)
Net cash provided by financing activities										40,480,000	25,354,000
Net change in cash and cash equivalents										10,203,000	(557,000)
Cash and cash equivalents at beginning of period					4,646,000				5,203,000	4,646,000	5,203,000	5,203,000
Cash and cash equivalents at end of period		14,849,000				4,646,000				14,849,000	4,646,000	14,849,000
Randolph Bancorp, Inc. and Predecessor Randolph Bancorp [Member]
Cash flows from operating activities:
Net income (loss)										463,000	(754,000)
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Equity in undistributed net (income) loss of Randolph Savings Bank										(2,797,000)	754,000
Charitable foundation contribution of Company stock										1,820,000
Net cash provided by (used in) operating activities										(514,000)
Cash flows from investing activities:
Acquisition of First Eastern, net of cash acquired										(11,131,000)
Investment in Randolph Savings Bank										(29,991,000)
ESOP loan										(4,695,000)
Payment received on ESOP loan										188,000
Net cash used in investing activities										(45,629,000)
Cash flows from financing activities:
Proceeds from issuance of common stock										56,868,000
Stock offering costs										(2,325,000)
Net cash provided by financing activities										54,543,000
Net change in cash and cash equivalents										8,400,000
Cash and cash equivalents at beginning of period					$ 96,000				$ 96,000	96,000	96,000	96,000
Cash and cash equivalents at end of period		$ 8,496,000				$ 96,000				8,496,000	$ 96,000	$ 8,496,000
Non-cash item:
Contribution of net assets of First Eastern to Randolph Savings Bank										$ 12,407,000