Parent-only Financial Information - Condensed Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
ASSETS:
Interest-earning deposits in other financial institutions	$ 132,409	$ 62,615
Other assets	26,212	31,586
Total assets	5,055,302	4,408,432
LIABILITIES AND STOCKHOLDERS’ EQUITY:
Total liabilities	4,359,449	3,741,993
Total stockholders' equity	695,853	666,439	$ 638,877	$ 621,196
Total liabilities and stockholders' equity	5,055,302	4,408,432
Parent Company
ASSETS:
Cash in Northfield Bank	32,828	21,884
Interest-earning deposits in other financial institutions	280	21
Investment in Northfield Bank	642,341	623,113
ESOP loan receivable	21,897	22,962
Other assets	132	120
Total assets	697,478	668,100
LIABILITIES AND STOCKHOLDERS’ EQUITY:
Total liabilities	1,625	1,661
Total stockholders' equity	695,853	666,439
Total liabilities and stockholders' equity	$ 697,478	$ 668,100