LOANS HELD FOR INVESTMENT, NET - Past Due Status (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	$ 9,284,871	$ 5,622,044
Non-accrual	34,020	17,928
Aggregate outstanding balance	6,300	3,300
Total Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	98,949	37,534
30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	51,138	15,359
60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	7,509	918
Loans 90 Days or More Past Due and Still Accruing Interest
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	6,282	3,329
Current
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	9,185,922	5,584,510
Total real estate loans
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	8,251,743	4,978,195
Non-accrual	9,919	5,425
Total real estate loans | Total Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	60,442	21,257
Total real estate loans | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	39,446	15,351
Total real estate loans | 60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	5,052	0
Total real estate loans | Loans 90 Days or More Past Due and Still Accruing Interest
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	6,025	481
Total real estate loans | Current
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	8,191,301	4,956,938
One-to-four family residential and cooperative/condominium apartment
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	683,665	184,989
Non-accrual	4,938	858
One-to-four family residential and cooperative/condominium apartment | Total Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	14,339	902
One-to-four family residential and cooperative/condominium apartment | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	3,135	0
One-to-four family residential and cooperative/condominium apartment | 60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	1,245	0
One-to-four family residential and cooperative/condominium apartment | Loans 90 Days or More Past Due and Still Accruing Interest
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	5,021	44
One-to-four family residential and cooperative/condominium apartment | Current
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	669,326	184,087
Multifamily residential and residential mixed-use
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	3,468,262	2,758,743
Non-accrual	859	1,863
Multifamily residential and residential mixed-use | Total Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	13,848	2,300
Multifamily residential and residential mixed-use | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	10,251	0
Multifamily residential and residential mixed-use | 60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	2,738	0
Multifamily residential and residential mixed-use | Loans 90 Days or More Past Due and Still Accruing Interest
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans		437
Multifamily residential and residential mixed-use | Current
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	3,454,414	2,756,443
Commercial real estate ("CRE")
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	3,814,437	1,878,167
Non-accrual	4,122	2,704
Commercial real estate ("CRE") | Total Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	14,555	18,055
Commercial real estate ("CRE") | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	8,360	15,351
Commercial real estate ("CRE") | 60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	1,069	0
Commercial real estate ("CRE") | Loans 90 Days or More Past Due and Still Accruing Interest
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	1,004	0
Commercial real estate ("CRE") | Current
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	3,799,882	1,860,112
Acquisition, development, and construction ("ADC")
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	285,379	156,296
Non-accrual	0	0
Acquisition, development, and construction ("ADC") | Total Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	17,700	0
Acquisition, development, and construction ("ADC") | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	17,700	0
Acquisition, development, and construction ("ADC") | 60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	0	0
Acquisition, development, and construction ("ADC") | Loans 90 Days or More Past Due and Still Accruing Interest
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	0	0
Acquisition, development, and construction ("ADC") | Current
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	267,679	156,296
Commercial and Industrial ("C&I") Loans
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	1,012,415	641,533
Non-accrual	23,727	12,502
Commercial and Industrial ("C&I") Loans | Total Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	37,401	16,267
Commercial and Industrial ("C&I") Loans | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	10,962	0
Commercial and Industrial ("C&I") Loans | 60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	2,455	917
Commercial and Industrial ("C&I") Loans | Loans 90 Days or More Past Due and Still Accruing Interest
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	257	2,848
Commercial and Industrial ("C&I") Loans | Current
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	975,014	625,266
Commercial and Industrial ("C&I") Loans | Total real estate loans
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans		5,619,728
Other Loans
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	20,713	2,316
Non-accrual	374	1
Other Loans | Total Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	1,106	10
Other Loans | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	730	8
Other Loans | 60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	2	1
Other Loans | Loans 90 Days or More Past Due and Still Accruing Interest
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	0	0
Other Loans | Current
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	$ 19,607	$ 2,306