LOANS HELD FOR INVESTMENT, NET - Credit Quality Indicators (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Credit Risk Profile of Real Estate Loans [Abstract]
Percentage of aggregate rental income			50.00%
Loans	$ 9,284,871		$ 9,284,871		$ 5,622,044
2021	1,329,531		1,329,531
2020	1,608,571		1,608,571
2019	1,354,530		1,354,530
2018	780,886		780,886
2017	954,652		954,652
2016 and Prior	2,577,987		2,577,987
Revolving	619,704		619,704
Revolving-Term	38,297		38,297
Total Loans	9,264,158		9,264,158
Pass
Credit Risk Profile of Real Estate Loans [Abstract]
2021	1,321,842		1,321,842
2020	1,582,785		1,582,785
2019	1,279,298		1,279,298
2018	684,132		684,132
2017	842,155		842,155
2016 and Prior	2,363,465		2,363,465
Revolving	578,714		578,714
Revolving-Term	28,110		28,110
Total Loans	8,680,501		8,680,501
Special Mention
Credit Risk Profile of Real Estate Loans [Abstract]
2021	5,354		5,354
2020	16,624		16,624
2019	15,153		15,153
2018	8,285		8,285
2017	23,907		23,907
2016 and Prior	40,023		40,023
Revolving	2,531		2,531
Revolving-Term	2,457		2,457
Total Loans	114,334		114,334
Substandard
Credit Risk Profile of Real Estate Loans [Abstract]
2021	2,335		2,335
2020	9,162		9,162
2019	49,886		49,886
2018	87,694		87,694
2017	76,601		76,601
2016 and Prior	174,469		174,469
Revolving	38,459		38,459
Revolving-Term	7,730		7,730
Total Loans	446,336		446,336
Doubtful
Credit Risk Profile of Real Estate Loans [Abstract]
2019	10,193		10,193
2018	775		775
2017	11,989		11,989
2016 and Prior	30		30
Total Loans	22,987		22,987
Criticized
Credit Risk Profile of Real Estate Loans [Abstract]
Sold amount	11,900	$ 3,000	66,500	$ 10,000
Total real estate loans
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	8,251,743		8,251,743		4,978,195
Total real estate loans | Pass
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					4,681,059
Total real estate loans | Special Mention
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					83,761
Total real estate loans | Substandard
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					213,375
Total real estate loans | Doubtful
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					0
One-to-four family residential and cooperative/condominium apartment
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	683,665		683,665		184,989
2021	104,830		104,830
2020	94,776		94,776
2019	88,599		88,599
2018	81,622		81,622
2017	86,626		86,626
2016 and Prior	160,666		160,666
Revolving	53,329		53,329
Revolving-Term	13,217		13,217
Total Loans	683,665		683,665
One-to-four family residential and cooperative/condominium apartment | Pass
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					183,293
2021	104,830		104,830
2020	93,315		93,315
2019	86,214		86,214
2018	79,981		79,981
2017	84,075		84,075
2016 and Prior	143,807		143,807
Revolving	52,483		52,483
Revolving-Term	11,162		11,162
Total Loans	655,867		655,867
One-to-four family residential and cooperative/condominium apartment | Special Mention
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					0
2019	337		337
2018	756		756
2017	345		345
2016 and Prior	2,168		2,168
Revolving	846		846
Revolving-Term	1,089		1,089
Total Loans	5,541		5,541
One-to-four family residential and cooperative/condominium apartment | Substandard
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					1,696
2020	1,461		1,461
2019	2,048		2,048
2018	862		862
2017	2,206		2,206
2016 and Prior	14,691		14,691
Revolving-Term	966		966
Total Loans	22,234		22,234
One-to-four family residential and cooperative/condominium apartment | Doubtful
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					0
2018	23		23
Total Loans	23		23
Multifamily residential and residential mixed-use
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	3,468,262		3,468,262		2,758,743
2021	426,164		426,164
2020	360,450		360,450
2019	543,514		543,514
2018	221,003		221,003
2017	437,487		437,487
2016 and Prior	1,470,245		1,470,245
Revolving	8,574		8,574
Revolving-Term	825		825
Total Loans	3,468,262		3,468,262
Multifamily residential and residential mixed-use | Pass
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					2,523,258
2021	426,164		426,164
2020	347,900		347,900
2019	493,077		493,077
2018	193,738		193,738
2017	373,965		373,965
2016 and Prior	1,329,688		1,329,688
Revolving	5,111		5,111
Revolving-Term	825		825
Total Loans	3,170,468		3,170,468
Multifamily residential and residential mixed-use | Special Mention
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					56,400
2020	12,550		12,550
2019	14,551		14,551
2017	11,842		11,842
2016 and Prior	22,378		22,378
Total Loans	61,321		61,321
Multifamily residential and residential mixed-use | Substandard
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					179,085
2019	35,886		35,886
2018	27,265		27,265
2017	51,680		51,680
2016 and Prior	118,179		118,179
Revolving	3,463		3,463
Total Loans	236,473		236,473
Multifamily residential and residential mixed-use | Doubtful
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					0
Commercial real estate ("CRE")
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	3,814,437		3,814,437		1,878,167
2021	643,923		643,923
2020	876,483		876,483
2019	585,974		585,974
2018	375,719		375,719
2017	368,772		368,772
2016 and Prior	919,079		919,079
Revolving	39,323		39,323
Revolving-Term	5,164		5,164
Total Loans	3,814,437		3,814,437
Commercial real estate ("CRE") | Pass
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					1,831,712
2021	636,234		636,234
2020	872,347		872,347
2019	578,758		578,758
2018	331,636		331,636
2017	337,930		337,930
2016 and Prior	863,215		863,215
Revolving	39,323		39,323
Revolving-Term	5,164		5,164
Total Loans	3,664,607		3,664,607
Commercial real estate ("CRE") | Special Mention
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					13,861
2021	5,354		5,354
2020	2,384		2,384
2018	4,191		4,191
2017	11,109		11,109
2016 and Prior	15,416		15,416
Total Loans	38,454		38,454
Commercial real estate ("CRE") | Substandard
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					32,594
2021	2,335		2,335
2020	1,752		1,752
2019	7,110		7,110
2018	39,892		39,892
2017	19,733		19,733
2016 and Prior	40,448		40,448
Total Loans	111,270		111,270
Commercial real estate ("CRE") | Doubtful
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					0
2019	106		106
Total Loans	106		106
Acquisition, development, and construction ("ADC")
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	285,379		285,379		156,296
2021	101,947		101,947
2020	69,546		69,546
2019	62,414		62,414
2018	39,165		39,165
2017	8,120		8,120
2016 and Prior	901		901
Revolving	2,686		2,686
Revolving-Term	600		600
Total Loans	285,379		285,379
Acquisition, development, and construction ("ADC") | Pass
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					142,796
2021	101,947		101,947
2020	69,546		69,546
2019	62,414		62,414
2018	24,587		24,587
2017	8,120		8,120
2016 and Prior	807		807
Revolving	2,686		2,686
Revolving-Term	600		600
Total Loans	270,707		270,707
Acquisition, development, and construction ("ADC") | Special Mention
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					13,500
2018	1,078		1,078
Total Loans	1,078		1,078
Acquisition, development, and construction ("ADC") | Substandard
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					0
2018	13,500		13,500
2016 and Prior	94		94
Total Loans	13,594		13,594
Acquisition, development, and construction ("ADC") | 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	1,012,415		1,012,415		641,533
2021	52,667		52,667
2020	207,316		207,316
2019	74,029		74,029
2018	63,377		63,377
2017	53,647		53,647
2016 and Prior	27,096		27,096
Revolving	515,792		515,792
Revolving-Term	18,491		18,491
Total Loans	1,012,415		1,012,415
Commercial and Industrial ("C&I") Loans | Pass
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					613,691
2021	52,667		52,667
2020	199,677		199,677
2019	58,835		58,835
2018	54,190		54,190
2017	38,065		38,065
2016 and Prior	25,948		25,948
Revolving	479,111		479,111
Revolving-Term	10,359		10,359
Total Loans	918,852		918,852
Commercial and Industrial ("C&I") Loans | Special Mention
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					2,131
2020	1,690		1,690
2019	265		265
2018	2,260		2,260
2017	611		611
2016 and Prior	61		61
Revolving	1,685		1,685
Revolving-Term	1,368		1,368
Total Loans	7,940		7,940
Commercial and Industrial ("C&I") Loans | Substandard
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					13,315
2020	5,949		5,949
2019	4,842		4,842
2018	6,175		6,175
2017	2,982		2,982
2016 and Prior	1,057		1,057
Revolving	34,996		34,996
Revolving-Term	6,764		6,764
Total Loans	62,765		62,765
Commercial and Industrial ("C&I") Loans | Doubtful
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					12,396
2019	10,087		10,087
2018	752		752
2017	11,989		11,989
2016 and Prior	30		30
Total Loans	22,858		22,858
Commercial and Industrial ("C&I") Loans | Total real estate loans
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					5,619,728
Commercial and Industrial ("C&I") Loans | Total real estate loans | Pass
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					5,294,750
Commercial and Industrial ("C&I") Loans | Total real estate loans | Special Mention
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					85,892
Commercial and Industrial ("C&I") Loans | Total real estate loans | Substandard
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					226,690
Commercial and Industrial ("C&I") Loans | Total real estate loans | Doubtful
Credit Risk Profile of Real Estate Loans [Abstract]
Loans					12,396
Other Loans
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	20,713		20,713		2,316
Other Loans | Credit Risk Profile
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	20,713		20,713
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	20,339		20,339
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	$ 374		$ 374