LOANS - Portfolio Allocated by Internal Risk Ratings (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	$ 6,281,039	$ 6,230,977
Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	5,772,742	6,119,022
Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	464,125	66,504
Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	33,434	33,343
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	10,738	10,508
Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	0	1,600
Multifamily residential
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	4,082,224	3,985,981
Multifamily residential | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	3,833,884	3,917,264
Multifamily residential | Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	223,406	47,309
Multifamily residential | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	20,739	19,708
Multifamily residential | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	4,195	1,700
Multifamily residential | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	0	0
Single family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	1,969,563	2,021,320
Single family residential | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	1,765,005	1,980,845
Single family residential | Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	185,320	16,432
Single family residential | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	12,695	13,635
Single family residential | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	6,543	8,808
Single family residential | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	0	1,600
Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	211,135	203,134
Commercial real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	155,736	200,371
Commercial real estate | Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	55,399	2,763
Commercial real estate | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	0	0
Commercial real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	0	0
Commercial real estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	0	0
Land, Construction and NM
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	18,117	20,542
Land, Construction and NM | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	18,117	20,542
Land, Construction and NM | Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	0	0
Land, Construction and NM | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	0	0
Land, Construction and NM | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	0	0
Land, Construction and NM | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	$ 0	$ 0