LOANS HELD FOR INVESTMENT, NET - Past Due (Details) $ in Thousands	Mar. 31, 2021 USD ($) loan		Dec. 31, 2020 USD ($) loan
Financing Receivable, Recorded Investment, Aging [Abstract]
Non-accrual	$ 35,549	[1]	$ 17,928
Total past due	86,211		37,534
Current	10,419,887		5,584,510
Loans	$ 10,506,098		$ 5,622,044
Number of loans receivables outstanding | loan	8		3
Aggregate outstanding balance	$ 8,800		$ 3,300
30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	28,976		15,359
60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	12,859		918
Loans 90 Days or More Past Due and Still Accruing Interest [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	8,827		3,329
Real estate
Financing Receivable, Recorded Investment, Aging [Abstract]
Non-accrual	20,927	[1]	5,425
Total past due	54,175		21,257
Current	8,094,904		4,956,938
Loans	8,149,079		4,978,195
Real estate | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	15,713		15,351
Real estate | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	12,360		0
Real estate | Loans 90 Days or More Past Due and Still Accruing Interest [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	5,175		481
Real estate | One-to-four family residential and cooperative/condominium apartment
Financing Receivable, Recorded Investment, Aging [Abstract]
Non-accrual	5,384	[1]	858
Total past due	13,142		902
Current	683,273		184,087
Loans	696,415		184,989
Real estate | One-to-four family residential and cooperative/condominium apartment | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	7,027		0
Real estate | One-to-four family residential and cooperative/condominium apartment | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	686		0
Real estate | One-to-four family residential and cooperative/condominium apartment | Loans 90 Days or More Past Due and Still Accruing Interest [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	45		44
Real estate | Multifamily residential and residential mixed-use
Financing Receivable, Recorded Investment, Aging [Abstract]
Non-accrual	4,844	[1]	1,863
Total past due	20,039		2,300
Current	3,547,168		2,756,443
Loans	3,567,207		2,758,743
Real estate | Multifamily residential and residential mixed-use | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	3,984		0
Real estate | Multifamily residential and residential mixed-use | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	8,340		0
Real estate | Multifamily residential and residential mixed-use | Loans 90 Days or More Past Due and Still Accruing Interest [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	2,871		437
Real estate | Commercial real estate ("CRE")
Financing Receivable, Recorded Investment, Aging [Abstract]
Non-accrual	10,595	[1]	2,704
Total past due	20,890		18,055
Current	3,610,397		1,860,112
Loans	3,631,287		1,878,167
Real estate | Commercial real estate ("CRE") | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	4,702		15,351
Real estate | Commercial real estate ("CRE") | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	3,334		0
Real estate | Commercial real estate ("CRE") | Loans 90 Days or More Past Due and Still Accruing Interest [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	2,259		0
Real estate | Acquisition, development, and construction ("ADC")
Financing Receivable, Recorded Investment, Aging [Abstract]
Non-accrual	104	[1]	0
Total past due	104		0
Current	254,066		156,296
Loans	254,170		156,296
Real estate | Acquisition, development, and construction ("ADC") | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	0		0
Real estate | Acquisition, development, and construction ("ADC") | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	0		0
Real estate | Acquisition, development, and construction ("ADC") | Loans 90 Days or More Past Due and Still Accruing Interest [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	0		0
Commercial and industrial ("C&I")
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	2,332,610		641,533
Commercial and industrial ("C&I"), including SBA and PPP loans
Financing Receivable, Recorded Investment, Aging [Abstract]
Non-accrual	14,523	[1]	12,502
Total past due	31,839		16,267
Current	2,300,771		625,266
Loans	2,332,610		641,533
Commercial and industrial ("C&I"), including SBA and PPP loans | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	13,254		0
Commercial and industrial ("C&I"), including SBA and PPP loans | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	410		917
Commercial and industrial ("C&I"), including SBA and PPP loans | Loans 90 Days or More Past Due and Still Accruing Interest [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	3,652		2,848
Other loans
Financing Receivable, Recorded Investment, Aging [Abstract]
Non-accrual	99	[1]	1
Total past due	197		10
Current	24,212		2,306
Loans	24,409		2,316
Other loans | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	9		8
Other loans | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	89		1
Other loans | Loans 90 Days or More Past Due and Still Accruing Interest [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	$ 0		$ 0

[1] Includes all loans on non-accrual status regardless of the number of days such loans were delinquent as of March 31, 2021.