LOANS HELD FOR INVESTMENT, NET - Credit Quality Indicators (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Credit Risk Profile of Real Estate Loans [Abstract]
Percentage of aggregate rental income	50.00%
Loans	$ 10,506,098	$ 5,622,044
2021	879,616
2020	2,373,222
2019	1,360,228
2018	830,392
2017	1,124,441
2016 and Prior	3,043,267
Revolving	724,519
Revolving-Term	146,004
Total	10,481,689
One-to-four family residential and cooperative/condominium apartment
Credit Risk Profile of Real Estate Loans [Abstract]
2021	30,658
2020	102,503
2019	97,118
2018	105,785
2017	101,472
2016 and Prior	188,043
Revolving	55,025
Revolving-Term	15,811
Total	696,415
One-to-four family residential and cooperative/condominium apartment | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
2021	30,658
2020	102,503
2019	95,992
2018	105,587
2017	101,101
2016 and Prior	177,780
Revolving	54,179
Revolving-Term	13,426
Total	681,226
One-to-four family residential and cooperative/condominium apartment | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
2019	381
2018	152
2017	371
2016 and Prior	5,106
Revolving	846
Revolving-Term	1,106
Total	7,962
One-to-four family residential and cooperative/condominium apartment | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
2019	745
2018	46
2016 and Prior	5,157
Revolving-Term	1,279
Total	7,227
Multifamily residential and residential mixed-use
Credit Risk Profile of Real Estate Loans [Abstract]
2021	95,876
2020	365,073
2019	554,328
2018	242,739
2017	514,157
2016 and Prior	1,784,829
Revolving	10,194
Revolving-Term	11
Total	3,567,207
Multifamily residential and residential mixed-use | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
2021	95,876
2020	365,073
2019	518,902
2018	215,459
2017	464,424
2016 and Prior	1,672,655
Revolving	9,698
Revolving-Term	11
Total	3,342,098
Multifamily residential and residential mixed-use | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
2019	1,005
2017	17,169
2016 and Prior	17,057
Total	35,231
Multifamily residential and residential mixed-use | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
2019	34,421
2018	27,280
2017	32,564
2016 and Prior	95,117
Revolving	496
Total	189,878
Commercial real estate ("CRE")
Credit Risk Profile of Real Estate Loans [Abstract]
2021	184,029
2020	872,325
2019	610,966
2018	419,212
2017	440,167
2016 and Prior	1,028,085
Revolving	50,050
Revolving-Term	26,453
Total	3,631,287
Commercial real estate ("CRE") | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
2021	184,029
2020	867,480
2019	604,893
2018	396,946
2017	404,571
2016 and Prior	978,940
Revolving	50,050
Revolving-Term	26,440
Total	3,513,349
Commercial real estate ("CRE") | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
2020	4,256
2019	2,673
2018	9,031
2017	13,536
2016 and Prior	11,578
Total	41,074
Commercial real estate ("CRE") | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
2020	589
2019	3,400
2018	13,235
2017	22,060
2016 and Prior	37,567
Revolving-Term	10
Total	76,861
Commercial real estate ("CRE") | Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Revolving-Term	3
Total	3
Acquisition, development, and construction ("ADC")
Credit Risk Profile of Real Estate Loans [Abstract]
2021	8,105
2020	39,106
2019	23,599
2018	11,597
2017	5,615
2016 and Prior	1,460
Revolving	121,537
Revolving-Term	43,151
Total	254,170
Acquisition, development, and construction ("ADC") | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
2021	8,105
2020	39,106
2019	23,599
2018	10,520
2017	5,615
2016 and Prior	1,355
Revolving	121,537
Revolving-Term	29,651
Total	239,488
Acquisition, development, and construction ("ADC") | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
2018	1,077
Revolving-Term	13,500
Total	14,577
Acquisition, development, and construction ("ADC") | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
2016 and Prior	105
Total	105
C&I [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
2021	560,948
2020	994,215
2019	74,217
2018	51,059
2017	63,030
2016 and Prior	40,850
Revolving	487,713
Revolving-Term	60,578
Total	2,332,610
C&I [Member] | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
2021	560,948
2020	991,821
2019	71,898
2018	44,089
2017	47,108
2016 and Prior	38,787
Revolving	461,029
Revolving-Term	42,082
Total	2,257,762
C&I [Member] | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
2020	2,345
2019	1,270
2018	5,553
2017	765
2016 and Prior	559
Revolving	22,566
Revolving-Term	3,078
Total	36,136
C&I [Member] | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
2020	49
2019	839
2018	1,417
2017	3,228
2016 and Prior	1,314
Revolving	4,118
Revolving-Term	15,190
Total	26,155
C&I [Member] | Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
2019	210
2017	11,929
2016 and Prior	190
Revolving-Term	228
Total	12,557
Real estate
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	8,149,079	4,978,195
Real estate | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		4,681,059
Real estate | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		83,761
Real estate | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		213,375
Real estate | Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		0
Real estate | One-to-four family residential and cooperative/condominium apartment
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	696,415	184,989
Real estate | One-to-four family residential and cooperative/condominium apartment | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		183,293
Real estate | One-to-four family residential and cooperative/condominium apartment | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		0
Real estate | One-to-four family residential and cooperative/condominium apartment | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		1,696
Real estate | One-to-four family residential and cooperative/condominium apartment | Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		0
Real estate | Multifamily residential and residential mixed-use
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	3,567,207	2,758,743
Real estate | Multifamily residential and residential mixed-use | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		2,523,258
Real estate | Multifamily residential and residential mixed-use | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		56,400
Real estate | Multifamily residential and residential mixed-use | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		179,085
Real estate | Multifamily residential and residential mixed-use | Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		0
Real estate | Commercial real estate ("CRE")
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	3,631,287	1,878,167
Real estate | Commercial real estate ("CRE") | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		1,831,712
Real estate | Commercial real estate ("CRE") | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		13,861
Real estate | Commercial real estate ("CRE") | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		32,594
Real estate | Commercial real estate ("CRE") | Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		0
Real estate | Acquisition, development, and construction ("ADC")
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	254,170	156,296
Real estate | Acquisition, development, and construction ("ADC") | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		142,796
Real estate | Acquisition, development, and construction ("ADC") | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		13,500
Real estate | Acquisition, development, and construction ("ADC") | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		0
Real estate | Acquisition, development, and construction ("ADC") | Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		0
Commercial and industrial ("C&I")
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	2,332,610	641,533
Commercial and industrial ("C&I"), including SBA and PPP loans
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	2,332,610	641,533
Commercial and industrial ("C&I"), including SBA and PPP loans | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		613,691
Commercial and industrial ("C&I"), including SBA and PPP loans | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		2,131
Commercial and industrial ("C&I"), including SBA and PPP loans | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		13,315
Commercial and industrial ("C&I"), including SBA and PPP loans | Doubtful [Member]
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 [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		5,294,750
Real Estate Loans and C & I Loans [Member] | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		85,892
Real Estate Loans and C & I Loans [Member] | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		226,690
Real Estate Loans and C & I Loans [Member] | Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		12,396
Other loans
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	24,409	2,316
Other loans | Credit Risk Profile [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	24,409
Other loans | Performing [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		2,315
Other loans | Performing [Member] | Credit Risk Profile [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	24,310
Other loans | Non-Accrual [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans		$ 1
Other loans | Non-Accrual [Member] | Credit Risk Profile [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	$ 99