Parent-only Financial Information - Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net income	$ 10,121	$ 13,139	$ 8,204	$ 8,771	$ 9,946	$ 9,076	$ 10,612	$ 10,445	$ 40,235	$ 40,079	$ 24,768
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
(Increase) decrease in other assets									2,679	3,563	(3,689)
Net cash (used in) provided by operating activities									51,152	52,789	43,624
Cash flows from investing activities:
Net cash provided by investing activities									(522,725)	(406,360)	(190,526)
Cash flows from financing activities:
Exercise of stock options									5,770	2,088	100
Net cash used in financing activities									541,629	373,494	108,656
Net increase (decrease) in cash and cash equivalents									70,056	19,923	(38,246)
Cash and cash equivalents at beginning of period				77,762				57,839	77,762	57,839	96,085
Cash and cash equivalents at end of period	147,818				77,762				147,818	77,762	57,839
Parent Company
Cash flows from operating activities:
Net income									40,235	40,079	24,768
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
(Increase) decrease in other assets									(1,073)	3	(2,008)
(Gains) losses on securities, net									(10)	6	(5)
(Decrease) increase in other liabilities									(36)	846	(980)
Undistributed earnings of Northfield Bank									(40,012)	(40,092)	(24,887)
Net cash (used in) provided by operating activities									(896)	842	(3,112)
Cash flows from investing activities:
Dividends from Northfield Bank									41,277	16,493	28,763
Net cash provided by investing activities									41,277	16,493	28,763
Cash flows from financing activities:
Principal payments on ESOP loan receivable									1,065	1,059	1,022
Purchase of treasury stock									(15,815)	(5)	0
Dividends paid									(20,198)	(18,673)	(15,646)
Exercise of stock options									5,770	2,088	100
Net cash used in financing activities									(29,178)	(15,531)	(14,524)
Net increase (decrease) in cash and cash equivalents									11,203	1,804	11,127
Cash and cash equivalents at beginning of period				$ 21,905				$ 20,101	21,905	20,101	8,974
Cash and cash equivalents at end of period	$ 33,108				$ 21,905				$ 33,108	$ 21,905	$ 20,101