Parent-only Financial Information - Condensed Statements of Comprehensive Income (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
Income tax expense	$ 3,052	$ 4,845	$ 2,280	$ 2,610	$ 2,314	$ 3,081	$ 1,893	$ 2,344	$ 12,787	$ 9,632	$ 26,978
Net income	$ 10,121	$ 13,139	$ 8,204	$ 8,771	$ 9,946	$ 9,076	$ 10,612	$ 10,445	40,235	40,079	24,768
Other comprehensive income (loss), net of tax									13,846	(3,696)	(227)
Comprehensive income									54,081	36,383	24,541
Parent Company
Interest on ESOP loan									1,263	1,081	939
Interest income on deposits in other financial institutions									229	38	31
Gains (losses) gains on securities, net									10	(6)	5
Undistributed earnings of Northfield Bank									40,012	40,092	24,887
Total income									41,514	41,205	25,862
Other expenses									863	890	872
Income tax expense									416	236	222
Total expense									1,279	1,126	1,094
Net income									40,235	40,079	24,768
Other comprehensive income (loss), net of tax									13,846	(3,696)	(227)
Comprehensive income									$ 54,081	$ 36,383	$ 24,541