LOANS - Portfolio Allocated by Internal Risk Ratings (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	$ 6,148,501	$ 6,230,977
Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	5,602,627	6,119,022
Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	497,887	66,504
Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	34,303	33,343
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	13,684	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	4,086,059	3,985,981
Multifamily residential | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	3,795,037	3,917,264
Multifamily residential | Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	263,250	47,309
Multifamily residential | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	19,155	19,708
Multifamily residential | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	8,617	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	1,839,156	2,021,320
Single family residential | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	1,650,400	1,980,845
Single family residential | Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	172,167	16,432
Single family residential | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	11,522	13,635
Single family residential | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	5,067	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	203,920	203,134
Commercial real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	137,824	200,371
Commercial real estate | Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	62,470	2,763
Commercial real estate | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	3,626	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	19,366	20,542
Land, Construction and NM | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, gross	19,366	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