Parent-only Financial Information - Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash flows from operating activities
Net income	$ 37,669	$ 61,119	$ 70,654
Adjustments to reconcile net income to net cash used in operating activities:
(Increase) decrease in other assets	(8,947)	(3,673)	(14,531)
Amortization of debt issuance costs	223	112	0
Net cash provided by operating activities	46,970	83,331	64,759
Cash flows from investing activities
Net cash provided by (used in) investing activities	193,869	(280,213)	84,995
Cash flows from financing activities
Proceeds from issuance of subordinated debt, net of issuance costs	0	60,884	0
Exercise of stock options	100	1,662	3,409
Net cash (used in) provided by financing activities	(57,132)	151,613	(146,230)
Net (decrease) increase in cash and cash equivalents	183,707	(45,269)	3,524
Cash and cash equivalents at beginning of year	45,799	91,068	87,544
Cash and cash equivalents at end of year	229,506	45,799	91,068
Parent Company
Cash flows from operating activities
Net income	37,669	61,119	70,654
Adjustments to reconcile net income to net cash used in operating activities:
(Increase) decrease in other assets	(3,158)	702	(4,034)
(Decrease) increase in other liabilities	(679)	420	12
Undistributed earnings of Northfield Bank	(39,662)	(62,964)	(70,956)
Net cash provided by operating activities	(5,607)	(611)	(4,324)
Cash flows from investing activities
Dividends from Northfield Bank	53,400	17,143	69,916
Net cash provided by (used in) investing activities	53,400	17,143	69,916
Cash flows from financing activities
Proceeds from issuance of subordinated debt, net of issuance costs	0	60,884	0
Principal payments on ESOP loan receivable	1,313	1,469	1,553
Purchase of treasury stock	(37,173)	(30,881)	(53,321)
Dividends paid	(22,795)	(24,127)	(24,299)
Exercise of stock options	100	1,662	3,409
Net cash (used in) provided by financing activities	(58,555)	9,007	(72,658)
Net (decrease) increase in cash and cash equivalents	(10,762)	25,539	(7,066)
Cash and cash equivalents at beginning of year	39,950	14,411	21,477
Cash and cash equivalents at end of year	$ 29,188	$ 39,950	$ 14,411