FEDERAL HOME LOAN BANK ADVANCES - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Feb. 01, 2021	Dec. 31, 2020
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Advances collateralized amount	$ 4,100,000,000		$ 4,100,000,000			$ 2,200,000,000
Maximum borrowing amount from FHLB term advances	3,800,000,000		3,800,000,000
FHLB advances with contractual maturities after 2021	0		0
Prepayments penalty expenses		$ 0	1,800,000	$ 0
Legacy Dime
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
FHLB advances					$ 216,300,000
Extinguishment of FHLB	$ 78,900,000		$ 209,000,000.0
Weighted average rate	0.33%		1.31%
Prepayments penalty expenses	$ 157,000