SUMMARY OF ACCOUNTING POLICIES (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Jan. 01, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	$ 81,255	$ 92,760		$ 41,461	$ 48,492	$ 42,492	$ 28,441
Retained earnings	630,744			600,641
Cumulative Effect, Period of Adoption, Adjustment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses				(3,919)
Accounting Standards Update 2016-13 | Cumulative Effect, Period of Adoption, Adjustment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	(3,900)
Reserve for unfunded commitments	1,400
Retained earnings			$ 1,700
One-to-four family residential and cooperative/condominium apartment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	$ 6,104	5,522		644	799	671	269
One-to-four family residential and cooperative/condominium apartment | Cumulative Effect, Period of Adoption, Adjustment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses				1,048
One-to-four family residential and cooperative/condominium apartment | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Value ratio	75.00%
One-to-four family residential and cooperative/condominium apartment | Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Debt service ratio	1.25%
Multifamily residential and residential mixed-use
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	$ 8,307	10,285		17,016	20,210	16,666	10,142
Multifamily residential and residential mixed-use | Cumulative Effect, Period of Adoption, Adjustment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses				(8,254)
Multifamily residential and residential mixed-use | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Value ratio	75.00%
Multifamily residential and residential mixed-use | Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Debt service ratio	1.20%
Commercial real estate ("CRE")
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	$ 29,603	41,201		9,059	12,021	9,859	3,900
Commercial real estate ("CRE") | Cumulative Effect, Period of Adoption, Adjustment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses				4,849
Commercial real estate ("CRE") | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Value ratio	75.00%
Commercial real estate ("CRE") | Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Debt service ratio	1.25%
Acquisition, Development, And Construction Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Value ratio	50.00%
Commercial and Industrial ("C&I") Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	$ 31,545	30,095		12,737	13,395	13,502	12,870
Value ratio	100.00%
Commercial and Industrial ("C&I") Loans | Cumulative Effect, Period of Adoption, Adjustment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses				(1,935)
Other Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	$ 677	$ 499		12	$ 16	$ 17	$ 16
Other Loans | Cumulative Effect, Period of Adoption, Adjustment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses				$ (8)