LOANS HELD FOR INVESTMENT, NET - Allowance for Credit Losses Activity (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Allowance for loan losses [Roll Forward]
Beginning balance	$ 92,760	$ 42,492	$ 41,461	$ 28,441
PCD Day 1			52,284
Provision (credit) for credit loss expense	(7,314)	5,931	812	20,003
Charge-offs	(4,276)	(20)	(9,584)	(62)
Recoveries	85	89	201	110
Ending balance	81,255	48,492	81,255	48,492
Non-accrual with No Allowance	2,099		2,099
Non-accrual with Allowance	31,921		31,921
Reserve			13,155
Interest income on non-accrual loans	0		0
Allowance for loan losses:
Individually evaluated for impairment					$ 6,474
Collectively evaluated for impairment					34,987
Total ending allowance balance	81,255	48,492	81,255	48,492	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)
Allowance for loan losses:
Total ending allowance balance					(3,919)
Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for loan losses [Roll Forward]
Beginning balance			37,542
Allowance for loan losses:
Total ending allowance balance					37,542
Total real estate loans
Allowance for loan losses [Roll Forward]
Beginning balance	62,166	28,973	28,712	15,555
PCD Day 1			28,753
Provision (credit) for credit loss expense	(10,203)	6,038	(2,689)	19,484
Charge-offs	(3,011)	(19)	(3,852)	(61)
Recoveries	81	89	85	103
Ending balance	49,033	35,081	49,033	35,081
Allowance for loan losses:
Collectively evaluated for impairment					28,712
Total ending allowance balance	49,033	35,081	49,033	35,081	28,712
Loans:
Individually evaluated for impairment					4,567
Collectively evaluated for impairment					4,973,628
Total ending loans balance					4,978,195
Total real estate loans | Cumulative Effect, Period of Adoption, Adjustment
Allowance for loan losses [Roll Forward]
Beginning balance			(1,976)
Allowance for loan losses:
Total ending allowance balance					(1,976)
Total real estate loans | Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for loan losses [Roll Forward]
Beginning balance			26,736
Allowance for loan losses:
Total ending allowance balance					26,736
One-to-four family residential and cooperative/condominium apartment
Allowance for loan losses [Roll Forward]
Beginning balance	5,522	671	644	269
PCD Day 1			2,220
Provision (credit) for credit loss expense	583	134	2,212	540
Charge-offs	(1)	(6)	(20)	(10)
Recoveries		0
Ending balance	6,104	799	6,104	799
Non-accrual with Allowance	4,938		4,938
Reserve			781
Allowance for loan losses:
Collectively evaluated for impairment					644
Total ending allowance balance	6,104	799	6,104	799	644
Loans:
Collectively evaluated for impairment					184,989
Total ending loans balance					184,989
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
Allowance for loan losses:
Total ending allowance balance					1,048
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
Allowance for loan losses:
Total ending allowance balance					1,692
Multifamily residential and residential mixed-use
Allowance for loan losses [Roll Forward]
Beginning balance	10,285	16,666	17,016	10,142
PCD Day 1			3,292
Provision (credit) for credit loss expense	(1,998)	3,468	(3,361)	10,010
Charge-offs	(58)	(13)	(467)	(45)
Recoveries	78	89	81	103
Ending balance	8,307	20,210	8,307	20,210
Non-accrual with No Allowance	859		859
Allowance for loan losses:
Collectively evaluated for impairment					17,016
Total ending allowance balance	8,307	20,210	8,307	20,210	17,016
Loans:
Individually evaluated for impairment					1,863
Collectively evaluated for impairment					2,756,880
Total ending loans balance					2,758,743
Multifamily residential and residential mixed-use | Cumulative Effect, Period of Adoption, Adjustment
Allowance for loan losses [Roll Forward]
Beginning balance			(8,254)
Allowance for loan losses:
Total ending allowance balance					(8,254)
Multifamily residential and residential mixed-use | Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for loan losses [Roll Forward]
Beginning balance			8,762
Allowance for loan losses:
Total ending allowance balance					8,762
Commercial real estate ("CRE")
Allowance for loan losses [Roll Forward]
Beginning balance	41,201	9,859	9,059	3,900
PCD Day 1			23,124
Provision (credit) for credit loss expense	(8,649)	2,162	(4,068)	8,127
Charge-offs	(2,952)	0	(3,365)	(6)
Recoveries	3	0	4
Ending balance	29,603	12,021	29,603	12,021
Non-accrual with No Allowance	1,240		1,240
Non-accrual with Allowance	2,882		2,882
Reserve			812
Allowance for loan losses:
Collectively evaluated for impairment					9,059
Total ending allowance balance	29,603	12,021	29,603	12,021	9,059
Loans:
Individually evaluated for impairment					2,704
Collectively evaluated for impairment					1,875,463
Total ending loans balance					1,878,167
Commercial real estate ("CRE") | Cumulative Effect, Period of Adoption, Adjustment
Allowance for loan losses [Roll Forward]
Beginning balance			4,849
Allowance for loan losses:
Total ending allowance balance					4,849
Commercial real estate ("CRE") | Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for loan losses [Roll Forward]
Beginning balance			13,908
Allowance for loan losses:
Total ending allowance balance					13,908
Acquisition, development, and construction ("ADC")
Allowance for loan losses [Roll Forward]
Beginning balance	5,158	1,777	1,993	1,244
PCD Day 1			117
Provision (credit) for credit loss expense	(139)	274	2,528	807
Charge-offs		0
Recoveries		0
Ending balance	5,019	2,051	5,019	2,051
Allowance for loan losses:
Collectively evaluated for impairment					1,993
Total ending allowance balance	5,019	2,051	5,019	2,051	1,993
Loans:
Collectively evaluated for impairment					156,296
Total ending loans balance					156,296
Acquisition, development, and construction ("ADC") | Cumulative Effect, Period of Adoption, Adjustment
Allowance for loan losses [Roll Forward]
Beginning balance			381
Allowance for loan losses:
Total ending allowance balance					381
Acquisition, development, and construction ("ADC") | Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for loan losses [Roll Forward]
Beginning balance			2,374
Allowance for loan losses:
Total ending allowance balance					2,374
Commercial and Industrial ("C&I") Loans
Allowance for loan losses [Roll Forward]
Beginning balance	30,095	13,502	12,737	12,870
PCD Day 1			23,374
Provision (credit) for credit loss expense	1,943	(107)	2,215	518
Charge-offs	(497)	0	(4,959)
Recoveries	4	0	113	7
Ending balance	31,545	13,395	31,545	13,395
Non-accrual with Allowance	23,727		23,727
Reserve			11,191
Allowance for loan losses:
Individually evaluated for impairment					6,474
Collectively evaluated for impairment					6,263
Total ending allowance balance	31,545	13,395	31,545	13,395	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") Loans | Cumulative Effect, Period of Adoption, Adjustment
Allowance for loan losses [Roll Forward]
Beginning balance			(1,935)
Allowance for loan losses:
Total ending allowance balance					(1,935)
Commercial and Industrial ("C&I") Loans | Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for loan losses [Roll Forward]
Beginning balance			10,802
Allowance for loan losses:
Total ending allowance balance					10,802
Other Loans
Allowance for loan losses [Roll Forward]
Beginning balance	499	17	12	16
PCD Day 1			157
Provision (credit) for credit loss expense	946		1,286	1
Charge-offs	(768)	(1)	(773)	(1)
Recoveries		0	3
Ending balance	677	16	677	16
Non-accrual with Allowance	374		374
Reserve			371
Allowance for loan losses:
Collectively evaluated for impairment					12
Total ending allowance balance	$ 677	$ 16	677	$ 16	12
Loans:
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)
Allowance for loan losses:
Total ending allowance balance					(8)
Other Loans | Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for loan losses [Roll Forward]
Beginning balance			$ 4
Allowance for loan losses:
Total ending allowance balance					$ 4