Parent Company Condensed Financial Statements - Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Cash and due from banks	$ 4,370	$ 2,721
Total assets	481,224	383,163
Stockholders' Equity:
Preferred stock
Common stock	59
Additional paid-in capital	56,373
Retained earnings	32,661	32,198
ESOP-Unearned compensation	(4,507)
Accumulated other comprehensive income (loss), net of tax	(1,284)	261
Total stockholders' equity	83,302	32,459	$ 33,656
Total liabilities and stockholders' equity	481,224	383,163
Randolph Bancorp, Inc. and Predecessor Randolph Bancorp [Member]
Assets
Cash and due from banks	8,496	96
Investment in Randolph Savings Bank	70,299	32,363
ESOP loan	4,507
Total assets	83,302	32,459
Stockholders' Equity:
Preferred stock
Common stock	59
Additional paid-in capital	56,373
Retained earnings	32,661	32,198
ESOP-Unearned compensation	(4,507)
Accumulated other comprehensive income (loss), net of tax	(1,284)	261
Total stockholders' equity	83,302	32,459
Total liabilities and stockholders' equity	$ 83,302	$ 32,459