Condensed Statement of Cash Flows of Parent Company (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net income	$ 4,603	$ 6,228	$ 6,068	$ 6,010	$ 2,108	$ 4,599	$ 4,012	$ 3,666	$ 22,909	$ 14,386	$ 11,981
Adjustments to reconcile net income to net cash used in operating activities:
Net cash provided by operating activities									59,083	85,765	12,773
Cash flows from financing activities:
Proceeds from exercise of stock options, net of cash paid									537	56	298
Net cash provided by financing activities									322,202	498,494	334,478
Net increase in cash and cash equivalents									32,490	52,012	7,615
Cash and cash equivalents at beginning of period				110,888				58,876	110,888	58,876	51,261
Cash and cash equivalents at end of period	143,378				110,888				143,378	110,888	58,876
Parent Company [Member]
Cash flows from operating activities:
Net income									22,909	14,386	11,981
Adjustments to reconcile net income to net cash used in operating activities:
Other, net										26	(27)
Net cash provided by operating activities									(160)	(110)	(157)
Cash flows from investing activities:
Net cash provided by investing activities										460
Cash flows from financing activities:
Proceeds from exercise of stock options, net of cash paid									681	50	298
Restricted stock awards issued, net of awards surrendered											(118)
Net cash provided by financing activities									681	50	180
Net increase in cash and cash equivalents									521	400	23
Cash and cash equivalents at beginning of period				$ 919				$ 519	919	519	496
Cash and cash equivalents at end of period	$ 1,440				$ 919				1,440	919	519
Parent Company [Member] \| Belmont Savings Bank Foundation [Member]
Adjustments to reconcile net income to net cash used in operating activities:
Equity in undistributed earnings of Belmont Savings Bank									(22,942)	(14,432)	(12,025)
Parent Company [Member] \| Belmont Savings Bank Funding Corporation [Member]
Adjustments to reconcile net income to net cash used in operating activities:
Equity in undistributed earnings of Belmont Savings Bank									$ (127)	(90)	$ (86)
Cash flows from investing activities:
Return of capital from BSB Funding Corp										$ 460

BSB Bancorp, Inc. 10-K