LOANS HELD FOR INVESTMENT, NET - Credit Quality Indicators (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Credit Risk Profile of Real Estate Loans [Abstract]
Percentage of aggregate rental income			50.00%
Loans	$ 9,546,631		$ 9,546,631		$ 5,622,044
Criticized
Credit Risk Profile of Real Estate Loans [Abstract]
Sold amount	50,700	$ 0	54,600	$ 7,100
One- to-Four Family Residential, Including Condominium and Cooperative Apartment [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
2021	86,022		86,022
2020	100,125		100,125
2019	91,701		91,701
2018	92,641		92,641
2017	90,171		90,171
2016 and Prior	174,052		174,052
Revolving	55,975		55,975
Revolving-Term	13,802		13,802
Total Loans	704,489		704,489
One- to-Four Family Residential, Including Condominium and Cooperative Apartment [Member] | Pass
Credit Risk Profile of Real Estate Loans [Abstract]
2021	86,022		86,022
2020	100,125		100,125
2019	90,239		90,239
2018	91,137		91,137
2017	89,074		89,074
2016 and Prior	160,866		160,866
Revolving	55,128		55,128
Revolving-Term	11,691		11,691
Total Loans	684,282		684,282
One- to-Four Family Residential, Including Condominium and Cooperative Apartment [Member] | Special Mention
Credit Risk Profile of Real Estate Loans [Abstract]
2019	379		379
2018	915		915
2017	355		355
2016 and Prior	5,299		5,299
Revolving	847		847
Revolving-Term	1,098		1,098
Total Loans	8,893		8,893
One- to-Four Family Residential, Including Condominium and Cooperative Apartment [Member] | Substandard
Credit Risk Profile of Real Estate Loans [Abstract]
2019	1,083		1,083
2018	589		589
2017	742		742
2016 and Prior	7,887		7,887
Revolving-Term	1,013		1,013
Total Loans	11,314		11,314
Multifamily Residential and Residential Mixed-Use [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
2021	234,199		234,199
2020	362,014		362,014
2019	552,409		552,409
2018	253,955		253,955
2017	455,259		455,259
2016 and Prior	1,636,602		1,636,602
Revolving	8,767		8,767
Total Loans	3,503,205		3,503,205
Multifamily Residential and Residential Mixed-Use [Member] | Pass
Credit Risk Profile of Real Estate Loans [Abstract]
2021	234,199		234,199
2020	362,014		362,014
2019	516,988		516,988
2018	226,705		226,705
2017	407,607		407,607
2016 and Prior	1,529,746		1,529,746
Revolving	5,339		5,339
Total Loans	3,282,598		3,282,598
Multifamily Residential and Residential Mixed-Use [Member] | Special Mention
Credit Risk Profile of Real Estate Loans [Abstract]
2017	18,329		18,329
2016 and Prior	14,244		14,244
Total Loans	32,573		32,573
Multifamily Residential and Residential Mixed-Use [Member] | Substandard
Credit Risk Profile of Real Estate Loans [Abstract]
2019	35,421		35,421
2018	27,250		27,250
2017	29,323		29,323
2016 and Prior	92,612		92,612
Revolving	3,428		3,428
Total Loans	188,034		188,034
Commercial Real Estate and Commercial Mixed-Use [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
2021	410,562		410,562
2020	870,816		870,816
2019	598,147		598,147
2018	398,723		398,723
2017	400,592		400,592
2016 and Prior	957,512		957,512
Revolving	39,993		39,993
Revolving-Term	4,986		4,986
Total Loans	3,681,331		3,681,331
Commercial Real Estate and Commercial Mixed-Use [Member] | Pass
Credit Risk Profile of Real Estate Loans [Abstract]
2021	410,562		410,562
2020	865,541		865,541
2019	590,972		590,972
2018	351,435		351,435
2017	365,870		365,870
2016 and Prior	912,770		912,770
Revolving	39,993		39,993
Revolving-Term	4,986		4,986
Total Loans	3,542,129		3,542,129
Commercial Real Estate and Commercial Mixed-Use [Member] | Special Mention
Credit Risk Profile of Real Estate Loans [Abstract]
2020	4,229		4,229
2019	1,673		1,673
2018	4,851		4,851
2017	11,190		11,190
2016 and Prior	11,444		11,444
Total Loans	33,387		33,387
Commercial Real Estate and Commercial Mixed-Use [Member] | Substandard
Credit Risk Profile of Real Estate Loans [Abstract]
2020	1,046		1,046
2019	5,502		5,502
2018	42,437		42,437
2017	23,532		23,532
2016 and Prior	33,298		33,298
Total Loans	105,815		105,815
ADC [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
2021	61,748		61,748
2020	60,895		60,895
2019	81,000		81,000
2018	58,484		58,484
2017	8,807		8,807
2016 and Prior	1,377		1,377
Revolving	17,538		17,538
Revolving-Term	613		613
Total Loans	290,462		290,462
ADC [Member] | Pass
Credit Risk Profile of Real Estate Loans [Abstract]
2021	61,748		61,748
2020	60,895		60,895
2019	81,000		81,000
2018	43,906		43,906
2017	8,807		8,807
2016 and Prior	1,279		1,279
Revolving	17,538		17,538
Revolving-Term	613		613
Total Loans	275,786		275,786
ADC [Member] | Special Mention
Credit Risk Profile of Real Estate Loans [Abstract]
2018	1,078		1,078
Total Loans	1,078		1,078
ADC [Member] | Substandard
Credit Risk Profile of Real Estate Loans [Abstract]
2018	13,500		13,500
2016 and Prior	98		98
Total Loans	13,598		13,598
C&I
Credit Risk Profile of Real Estate Loans [Abstract]
2021	41,557		41,557
2020	557,766		557,766
2019	78,048		78,048
2018	70,415		70,415
2017	55,845		55,845
2016 and Prior	30,797		30,797
Revolving	495,152		495,152
Revolving-Term	14,289		14,289
Total Loans	1,343,869		1,343,869
C&I | Pass
Credit Risk Profile of Real Estate Loans [Abstract]
2021	41,557		41,557
2020	549,694		549,694
2019	63,141		63,141
2018	60,724		60,724
2017	40,088		40,088
2016 and Prior	29,378		29,378
Revolving	466,245		466,245
Revolving-Term	7,067		7,067
Total Loans	1,257,894		1,257,894
C&I | Special Mention
Credit Risk Profile of Real Estate Loans [Abstract]
2020	3,515		3,515
2019	291		291
2018	2,241		2,241
2017	643		643
2016 and Prior	116		116
Revolving	6,127		6,127
Revolving-Term	2,637		2,637
Total Loans	15,570		15,570
C&I | Substandard
Credit Risk Profile of Real Estate Loans [Abstract]
2020	4,557		4,557
2019	14,419		14,419
2018	6,698		6,698
2017	3,185		3,185
2016 and Prior	1,171		1,171
Revolving	22,780		22,780
Revolving-Term	4,585		4,585
Total Loans	57,395		57,395
C&I | Doubtful
Credit Risk Profile of Real Estate Loans [Abstract]
2019	197		197
2018	752		752
2017	11,929		11,929
2016 and Prior	132		132
Total Loans	13,010		13,010
Real estate
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	8,179,487		8,179,487		4,978,195
Real estate | Pass
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					4,681,059
Real estate | Special Mention
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					83,761
Real estate | Substandard
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					213,375
Real estate | Doubtful
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					0
Real estate | One- to-Four Family Residential, Including Condominium and Cooperative Apartment [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	704,489		704,489		184,989
Real estate | One- to-Four Family Residential, Including Condominium and Cooperative Apartment [Member] | Pass
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					183,293
Real estate | One- to-Four Family Residential, Including Condominium and Cooperative Apartment [Member] | Special Mention
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					0
Real estate | One- to-Four Family Residential, Including Condominium and Cooperative Apartment [Member] | Substandard
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					1,696
Real estate | One- to-Four Family Residential, Including Condominium and Cooperative Apartment [Member] | Doubtful
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					0
Real estate | Multifamily Residential and Residential Mixed-Use [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	3,503,205		3,503,205		2,758,743
Real estate | Multifamily Residential and Residential Mixed-Use [Member] | Pass
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					2,523,258
Real estate | Multifamily Residential and Residential Mixed-Use [Member] | Special Mention
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					56,400
Real estate | Multifamily Residential and Residential Mixed-Use [Member] | Substandard
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					179,085
Real estate | Multifamily Residential and Residential Mixed-Use [Member] | Doubtful
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					0
Real estate | Commercial Real Estate and Commercial Mixed-Use [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	3,681,331		3,681,331		1,878,167
Real estate | Commercial Real Estate and Commercial Mixed-Use [Member] | Pass
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					1,831,712
Real estate | Commercial Real Estate and Commercial Mixed-Use [Member] | Special Mention
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					13,861
Real estate | Commercial Real Estate and Commercial Mixed-Use [Member] | Substandard
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					32,594
Real estate | Commercial Real Estate and Commercial Mixed-Use [Member] | Doubtful
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					0
Real estate | ADC [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	290,462		290,462		156,296
Real estate | ADC [Member] | Pass
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					142,796
Real estate | ADC [Member] | Special Mention
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					13,500
Real estate | ADC [Member] | Substandard
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					0
Real estate | ADC [Member] | Doubtful
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					0
Commercial and industrial ("C&I") loans
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	1,343,869		1,343,869		641,533
Commercial and industrial ("C&I"), including SBA and PPP loans
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	1,343,869		1,343,869		641,533
Commercial and industrial ("C&I"), including SBA and PPP loans | Pass
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					613,691
Commercial and industrial ("C&I"), including SBA and PPP loans | Special Mention
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					2,131
Commercial and industrial ("C&I"), including SBA and PPP loans | Substandard
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					13,315
Commercial and industrial ("C&I"), including SBA and PPP loans | Doubtful
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					12,396
Real Estate Loans and C & I Loans [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					5,619,728
Real Estate Loans and C & I Loans [Member] | Pass
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					5,294,750
Real Estate Loans and C & I Loans [Member] | Special Mention
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					85,892
Real Estate Loans and C & I Loans [Member] | Substandard
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					226,690
Real Estate Loans and C & I Loans [Member] | Doubtful
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					12,396
Other loans
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	23,275		23,275		2,316
2021	834,088		834,088
2020	1,951,616		1,951,616
2019	1,401,305		1,401,305
2018	874,218		874,218
2017	1,010,674		1,010,674
2016 and Prior	2,800,340		2,800,340
Revolving	617,425		617,425
Revolving-Term	33,690		33,690
Total Loans	9,523,356		9,523,356
Other loans | Credit Risk Profile
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	23,275		23,275
Other loans | Performing
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					2,315
Other loans | Performing | Credit Risk Profile
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	23,183		23,183
Other loans | Non-Accrual
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					$ 1
Other loans | Non-Accrual | Credit Risk Profile
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	92		92
Other loans | Pass
Credit Risk Profile of Real Estate Loans [Abstract]
2021	834,088		834,088
2020	1,938,269		1,938,269
2019	1,342,340		1,342,340
2018	773,907		773,907
2017	911,446		911,446
2016 and Prior	2,634,039		2,634,039
Revolving	584,243		584,243
Revolving-Term	24,357		24,357
Total Loans	9,042,689		9,042,689
Other loans | Special Mention
Credit Risk Profile of Real Estate Loans [Abstract]
2020	7,744		7,744
2019	2,343		2,343
2018	9,085		9,085
2017	30,517		30,517
2016 and Prior	31,103		31,103
Revolving	6,974		6,974
Revolving-Term	3,735		3,735
Total Loans	91,501		91,501
Other loans | Substandard
Credit Risk Profile of Real Estate Loans [Abstract]
2020	5,603		5,603
2019	56,425		56,425
2018	90,474		90,474
2017	56,782		56,782
2016 and Prior	135,066		135,066
Revolving	26,208		26,208
Revolving-Term	5,598		5,598
Total Loans	376,156		376,156
Other loans | Doubtful
Credit Risk Profile of Real Estate Loans [Abstract]
2019	197		197
2018	752		752
2017	11,929		11,929
2016 and Prior	132		132
Total Loans	$ 13,010		$ 13,010