LOANS HELD FOR INVESTMENT, NET - Past Due Status (Details) $ in Thousands	Jun. 30, 2021 USD ($) loan		Dec. 31, 2020 USD ($) loan
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	$ 9,546,631		$ 5,622,044
Non-accrual	$ 28,286	[1]	$ 17,928
Number of loans receivables outstanding | loan	11		3
Aggregate outstanding balance	$ 6,700		$ 3,300
Total Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	193,503		37,534
30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	147,716		15,359
60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	10,791		918
Loans 90 Days or More Past Due and Still Accruing Interest
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	6,710		3,329
Current
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	9,353,128		5,584,510
Real estate
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	8,179,487		4,978,195
Non-accrual	14,085	[1]	5,425
Real estate | Total Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	176,829		21,257
Real estate | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	147,202		15,351
Real estate | 60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	10,319		0
Real estate | Loans 90 Days or More Past Due and Still Accruing Interest
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	5,223		481
Real estate | Current
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	8,002,658		4,956,938
Real estate | One- to-Four Family Residential, Including Condominium and Cooperative Apartment [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	704,489		184,989
Non-accrual	4,933	[1]	858
Real estate | One- to-Four Family Residential, Including Condominium and Cooperative Apartment [Member] | Total Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	17,975		902
Real estate | One- to-Four Family Residential, Including Condominium and Cooperative Apartment [Member] | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	3,852		0
Real estate | One- to-Four Family Residential, Including Condominium and Cooperative Apartment [Member] | 60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	4,124		0
Real estate | One- to-Four Family Residential, Including Condominium and Cooperative Apartment [Member] | Loans 90 Days or More Past Due and Still Accruing Interest
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	5,066		44
Real estate | One- to-Four Family Residential, Including Condominium and Cooperative Apartment [Member] | Current
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	686,514		184,087
Real estate | Multifamily Residential and Residential Mixed-Use [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	3,503,205		2,758,743
Non-accrual			1,863
Real estate | Multifamily Residential and Residential Mixed-Use [Member] | Total Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	114,136		2,300
Real estate | Multifamily Residential and Residential Mixed-Use [Member] | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	110,712		0
Real estate | Multifamily Residential and Residential Mixed-Use [Member] | 60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	3,267		0
Real estate | Multifamily Residential and Residential Mixed-Use [Member] | Loans 90 Days or More Past Due and Still Accruing Interest
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	157		437
Real estate | Multifamily Residential and Residential Mixed-Use [Member] | Current
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	3,389,069		2,756,443
Real estate | Commercial Real Estate and Commercial Mixed-Use [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	3,681,331		1,878,167
Non-accrual	9,152	[1]	2,704
Real estate | Commercial Real Estate and Commercial Mixed-Use [Member] | Total Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	44,718		18,055
Real estate | Commercial Real Estate and Commercial Mixed-Use [Member] | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	32,638		15,351
Real estate | Commercial Real Estate and Commercial Mixed-Use [Member] | 60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	2,928		0
Real estate | Commercial Real Estate and Commercial Mixed-Use [Member] | Loans 90 Days or More Past Due and Still Accruing Interest
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans			0
Real estate | Commercial Real Estate and Commercial Mixed-Use [Member] | Current
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	3,636,613		1,860,112
Real estate | ADC [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	290,462		156,296
Non-accrual	0	[1]	0
Real estate | ADC [Member] | Total Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	0		0
Real estate | ADC [Member] | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	0		0
Real estate | ADC [Member] | 60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	0		0
Real estate | ADC [Member] | Loans 90 Days or More Past Due and Still Accruing Interest
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	0		0
Real estate | ADC [Member] | Current
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	290,462		156,296
Commercial and industrial ("C&I") loans
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	1,343,869		641,533
Commercial and industrial ("C&I"), including SBA and PPP loans
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	1,343,869		641,533
Non-accrual	14,109	[1]	12,502
Commercial and industrial ("C&I"), including SBA and PPP loans | Total Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	16,574		16,267
Commercial and industrial ("C&I"), including SBA and PPP loans | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	506		0
Commercial and industrial ("C&I"), including SBA and PPP loans | 60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	472		917
Commercial and industrial ("C&I"), including SBA and PPP loans | Loans 90 Days or More Past Due and Still Accruing Interest
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	1,487		2,848
Commercial and industrial ("C&I"), including SBA and PPP loans | Current
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	1,327,295		625,266
Other loans
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	23,275		2,316
Non-accrual	92	[1]	1
Other loans | Total Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	100		10
Other loans | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	8		8
Other loans | 60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	0		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	$ 23,175		$ 2,306

[1] Includes all loans on non-accrual status regardless of the number of days such loans were delinquent as of June 30, 2021.