FAIR VALUE OF FINANCIAL INSTRUMENTS - Balance Sheet Groupings (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Carrying Amount
Financial Assets [Abstract]
Cash and due from banks	$ 629,011	$ 243,603
Investment securities held-to-maturity, fair value	40,303
Loans held for investment, net	9,148,470	5,580,583
Accrued interest receivable	43,284	34,815
Financial Liabilities [Abstract]
Savings, money market and checking accounts	9,657,821	3,202,484
Certificates of Deposits ("CDs")	1,016,216	1,322,638
FHLBNY Advances	25,000	1,204,010
Subordinated debt, net	197,142	114,052
Other short-term borrowings	2,629	120,000
Accrued interest payable	1,630	1,734
Fair Value
Financial Assets [Abstract]
Cash and due from banks	629,011	243,603
Investment securities held-to-maturity, fair value	40,303
Loans held for investment, net	9,212,454	5,598,787
Accrued interest receivable	43,284	34,815
Financial Liabilities [Abstract]
Savings, money market and checking accounts	9,657,821	3,202,484
Certificates of Deposits ("CDs")	1,021,029	1,328,554
FHLBNY Advances	25,015	1,207,890
Subordinated debt, net	203,355	114,340
Other short-term borrowings	2,629	120,000
Accrued interest payable	1,630	1,734
Fair Value | Level 1 Inputs [Member]
Financial Assets [Abstract]
Cash and due from banks	629,011	243,603
Accrued interest receivable		2
Financial Liabilities [Abstract]
Savings, money market and checking accounts	9,657,821	3,202,484
Other short-term borrowings	2,629	120,000
Fair Value | Level 2 Inputs [Member]
Financial Assets [Abstract]
Investment securities held-to-maturity, fair value	40,303
Accrued interest receivable	4,228	1,584
Financial Liabilities [Abstract]
Certificates of Deposits ("CDs")	1,021,029	1,328,554
FHLBNY Advances	25,015	1,207,890
Subordinated debt, net	203,355	114,340
Accrued interest payable	1,630	1,734
Fair Value | Level 3 Inputs [Member]
Financial Assets [Abstract]
Loans held for investment, net	9,212,454	5,598,787
Accrued interest receivable	$ 39,056	$ 33,229