LOANS HELD FOR INVESTMENT, NET - Allowance by Class of Loan (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Mar. 31, 2021	Dec. 31, 2020
Allowance for loan losses [Roll Forward]
Beginning balance	$ 41,461	$ 28,441	$ 28,441
PCD Day 1	52,284
Provision (credit) for credit losses	12,649	8,012
Charge-offs	(4,275)	(6)
Recoveries		16
Ending balance	98,200	36,463	41,461
Non-accrual with No Allowance for Credit Loss				$ 8,396
Non-accrual with Allowance				27,153
Reserve	10,918
Interest income on non-accrual loans	0
CECL	3,900
Non-PCD	20,300
Impact of improvement in forecasted macroeconomic conditions			(7,600)
Allowance for loan losses:
Individually evaluated for impairment					$ 6,474
Collectively evaluated for impairment					34,987
Total ending allowance balance	98,200	36,463	28,441	98,200	41,461
Loans:
Individually evaluated for impairment					17,069
Collectively evaluated for impairment					5,604,975
Total ending loans balance					5,622,044
Cumulative Effect, Period of Adoption, Adjustment
Allowance for loan losses [Roll Forward]
Beginning balance	(3,919)
Ending balance			(3,919)
Allowance for loan losses:
Total ending allowance balance	(3,919)		(3,919)		(3,919)
Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for loan losses [Roll Forward]
Beginning balance	37,542
Ending balance			37,542
Allowance for loan losses:
Total ending allowance balance	37,542		37,542		37,542
One-to-four family residential and cooperative/condominium apartment
Allowance for loan losses [Roll Forward]
Non-accrual with Allowance				5,384
Reserve	134
Multifamily residential and residential mixed-use
Allowance for loan losses [Roll Forward]
Non-accrual with No Allowance for Credit Loss				4,844
Commercial real estate ("CRE")
Allowance for loan losses [Roll Forward]
Non-accrual with No Allowance for Credit Loss				3,552
Non-accrual with Allowance				7,043
Reserve	2,786
Acquisition, development, and construction ("ADC")
Allowance for loan losses [Roll Forward]
Non-accrual with Allowance				104
Reserve	59
Real estate
Allowance for loan losses [Roll Forward]
Beginning balance	28,712	15,555	15,555
PCD Day 1	28,753
Provision (credit) for credit losses	9,059	7,372
Charge-offs	(258)	(6)
Recoveries		14
Ending balance	64,290	22,935	28,712
Allowance for loan losses:
Individually evaluated for impairment					0
Collectively evaluated for impairment					28,712
Total ending allowance balance	64,290	22,935	15,555	64,290	28,712
Loans:
Individually evaluated for impairment					4,567
Collectively evaluated for impairment					4,973,628
Total ending loans balance					4,978,195
Real estate | Cumulative Effect, Period of Adoption, Adjustment
Allowance for loan losses [Roll Forward]
Beginning balance	(1,976)
Ending balance			(1,976)
Allowance for loan losses:
Total ending allowance balance	(1,976)		(1,976)		(1,976)
Real estate | Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for loan losses [Roll Forward]
Beginning balance	26,736
Ending balance			26,736
Allowance for loan losses:
Total ending allowance balance	26,736		26,736		26,736
Real estate | One-to-four family residential and cooperative/condominium apartment
Allowance for loan losses [Roll Forward]
Beginning balance	644	269	269
PCD Day 1	2,220
Provision (credit) for credit losses	1,235	376
Charge-offs	(14)	0
Recoveries		0
Ending balance	5,133	645	644
Allowance for loan losses:
Individually evaluated for impairment					0
Collectively evaluated for impairment					644
Total ending allowance balance	5,133	645	269	5,133	644
Loans:
Individually evaluated for impairment					0
Collectively evaluated for impairment					184,989
Total ending loans balance					184,989
Real estate | One-to-four family residential and cooperative/condominium apartment | Cumulative Effect, Period of Adoption, Adjustment
Allowance for loan losses [Roll Forward]
Beginning balance	1,048
Ending balance			1,048
Allowance for loan losses:
Total ending allowance balance	1,048		1,048		1,048
Real estate | One-to-four family residential and cooperative/condominium apartment | Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for loan losses [Roll Forward]
Beginning balance	1,692
Ending balance			1,692
Allowance for loan losses:
Total ending allowance balance	1,692		1,692		1,692
Real estate | Multifamily residential and residential mixed-use
Allowance for loan losses [Roll Forward]
Beginning balance	17,016	10,142	10,142
PCD Day 1	3,292
Provision (credit) for credit losses	(1,397)	4,127
Charge-offs	(236)	0
Recoveries		14
Ending balance	10,421	14,283	17,016
Allowance for loan losses:
Individually evaluated for impairment					0
Collectively evaluated for impairment					17,016
Total ending allowance balance	10,421	14,283	10,142	10,421	17,016
Loans:
Individually evaluated for impairment					1,863
Collectively evaluated for impairment					2,756,880
Total ending loans balance					2,758,743
Real estate | Multifamily residential and residential mixed-use | Cumulative Effect, Period of Adoption, Adjustment
Allowance for loan losses [Roll Forward]
Beginning balance	(8,254)
Ending balance			(8,254)
Allowance for loan losses:
Total ending allowance balance	(8,254)		(8,254)		(8,254)
Real estate | Multifamily residential and residential mixed-use | Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for loan losses [Roll Forward]
Beginning balance	8,762
Ending balance			8,762
Allowance for loan losses:
Total ending allowance balance	8,762		8,762		8,762
Real estate | Commercial real estate ("CRE")
Allowance for loan losses [Roll Forward]
Beginning balance	9,059	3,900	3,900
PCD Day 1	23,124
Provision (credit) for credit losses	7,813	2,442
Charge-offs	(8)	(6)
Recoveries		0
Ending balance	44,837	6,336	9,059
Allowance for loan losses:
Individually evaluated for impairment					0
Collectively evaluated for impairment					9,059
Total ending allowance balance	44,837	6,336	3,900	44,837	9,059
Loans:
Individually evaluated for impairment					2,704
Collectively evaluated for impairment					1,875,463
Total ending loans balance					1,878,167
Real estate | Commercial real estate ("CRE") | Cumulative Effect, Period of Adoption, Adjustment
Allowance for loan losses [Roll Forward]
Beginning balance	4,849
Ending balance			4,849
Allowance for loan losses:
Total ending allowance balance	4,849		4,849		4,849
Real estate | Commercial real estate ("CRE") | Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for loan losses [Roll Forward]
Beginning balance	13,908
Ending balance			13,908
Allowance for loan losses:
Total ending allowance balance	13,908		13,908		13,908
Real estate | Acquisition, development, and construction ("ADC")
Allowance for loan losses [Roll Forward]
Beginning balance	1,993	1,244	1,244
PCD Day 1	117
Provision (credit) for credit losses	1,408	427
Charge-offs		0
Recoveries		0
Ending balance	3,899	1,671	1,993
Allowance for loan losses:
Individually evaluated for impairment					0
Collectively evaluated for impairment					1,993
Total ending allowance balance	3,899	1,671	1,244	3,899	1,993
Loans:
Individually evaluated for impairment					0
Collectively evaluated for impairment					156,296
Total ending loans balance					156,296
Real estate | Acquisition, development, and construction ("ADC") | Cumulative Effect, Period of Adoption, Adjustment
Allowance for loan losses [Roll Forward]
Beginning balance	381
Ending balance			381
Allowance for loan losses:
Total ending allowance balance	381		381		381
Real estate | Acquisition, development, and construction ("ADC") | Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for loan losses [Roll Forward]
Beginning balance	2,374
Ending balance			2,374
Allowance for loan losses:
Total ending allowance balance	2,374		2,374		2,374
Commercial and industrial ("C&I")
Allowance for loan losses [Roll Forward]
Beginning balance	12,737	12,870	12,870
Provision (credit) for credit losses		641
Charge-offs		0
Recoveries		2
Ending balance		13,513	12,737
Allowance for loan losses:
Individually evaluated for impairment					6,474
Collectively evaluated for impairment					6,263
Total ending allowance balance	12,737	13,513	12,737		12,737
Loans:
Individually evaluated for impairment					12,502
Collectively evaluated for impairment					629,031
Total ending loans balance					641,533
Commercial and industrial ("C&I"), including SBA and PPP loans
Allowance for loan losses [Roll Forward]
Beginning balance	12,737
PCD Day 1	23,374
Provision (credit) for credit losses	3,219
Charge-offs	(4,017)
Ending balance	33,378		12,737
Non-accrual with Allowance				14,523
Reserve	7,881
Allowance for loan losses:
Total ending allowance balance	33,378		12,737	33,378	12,737
Commercial and industrial ("C&I"), including SBA and PPP loans | Cumulative Effect, Period of Adoption, Adjustment
Allowance for loan losses [Roll Forward]
Beginning balance	(1,935)
Ending balance			(1,935)
Allowance for loan losses:
Total ending allowance balance	(1,935)		(1,935)		(1,935)
Commercial and industrial ("C&I"), including SBA and PPP loans | Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for loan losses [Roll Forward]
Beginning balance	10,802
Ending balance			10,802
Allowance for loan losses:
Total ending allowance balance	10,802		10,802		10,802
Other loans
Allowance for loan losses [Roll Forward]
Beginning balance	12	16	16
PCD Day 1	157
Provision (credit) for credit losses	371	(1)
Charge-offs		0
Recoveries		0
Ending balance	532	15	12
Non-accrual with Allowance				99
Reserve	58
Allowance for loan losses:
Individually evaluated for impairment					0
Collectively evaluated for impairment					12
Total ending allowance balance	532	$ 15	16	$ 532	12
Loans:
Individually evaluated for impairment					0
Collectively evaluated for impairment					2,316
Total ending loans balance					2,316
Other loans | Cumulative Effect, Period of Adoption, Adjustment
Allowance for loan losses [Roll Forward]
Beginning balance	(8)
Ending balance			(8)
Allowance for loan losses:
Total ending allowance balance	(8)		(8)		(8)
Other loans | Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for loan losses [Roll Forward]
Beginning balance	4
Ending balance			4
Allowance for loan losses:
Total ending allowance balance	$ 4		$ 4		$ 4