Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	Charitable Foundation	Blue Foundry Bank ESOP	Common Stock	Common Stock Charitable Foundation	Additional Paid-In Capital	Additional Paid-In Capital Charitable Foundation	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Balance at beginning of period at Dec. 31, 2019	$ 237,621			$ 10		$ 822		$ 237,306	$ (517)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(11,334)							(11,334)
Other comprehensive income	(2,931)								(2,931)
Balance at end of period at Mar. 31, 2020	223,356			10		822		225,972	(3,448)
Balance at beginning of period at Dec. 31, 2019	237,621			10		822		237,306	(517)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(29,744)
Other comprehensive income	(1,787)
Balance at end of period at Sep. 30, 2020	206,091			10		822		207,563	(2,304)
Balance at beginning of period at Mar. 31, 2020	223,356			10		822		225,972	(3,448)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(16,744)							(16,744)
Other comprehensive income	820								820
Balance at end of period at Jun. 30, 2020	207,432			10		822		209,228	(2,628)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(1,665)							(1,665)
Other comprehensive income	324
Balance at end of period at Sep. 30, 2020	206,091			10		822		207,563	(2,304)
Balance at beginning of period at Dec. 31, 2020	205,600			10		822		205,799	(1,031)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(745)							(745)
Other comprehensive income	597								597
Balance at end of period at Mar. 31, 2021	205,452			10		822		205,054	(434)
Balance at beginning of period at Dec. 31, 2020	205,600			10		822		205,799	(1,031)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(16,718)
Other comprehensive income	519
Balance at end of period at Sep. 30, 2021	448,235		$ (22,405)	285		281,786		189,081	(512)
Balance at beginning of period at Mar. 31, 2021	205,452			10		822		205,054	(434)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(1,003)							(1,003)
Other comprehensive income	463								463
Balance at end of period at Jun. 30, 2021	204,912		0	10		822		204,051	29
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(14,970)							(14,970)
Proceeds from stock issued during the period	273,598	$ 7,500		268	$ 7	273,330	$ 7,493
Stock offering costs incurred and deferred	4,800
Purchase of common shares by the ESOP (2,281,800 shares)	(22,818)		(22,818)
ESOP shares committed to be released	554		413			141
Other comprehensive income	(541)								(541)
Balance at end of period at Sep. 30, 2021	$ 448,235		$ (22,405)	$ 285		$ 281,786		$ 189,081	$ (512)