LOANS - Portfolio Allocated by Internal Risk Ratings (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Total	$ 6,218,314	$ 6,230,977
Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total	6,115,008	6,119,022
Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Total	68,343	66,504
Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total	26,800	33,343
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total	8,163	10,508
Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	1,600
Multifamily residential
Financing Receivable, Credit Quality Indicator [Line Items]
Total	4,058,869	3,985,981
Multifamily residential | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total	3,989,553	3,917,264
Multifamily residential | Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Total	51,699	47,309
Multifamily residential | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total	16,692	19,708
Multifamily residential | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total	925	1,700
Multifamily residential | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Single family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Total	1,930,831	2,021,320
Single family residential | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total	1,898,641	1,980,845
Single family residential | Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Total	14,844	16,432
Single family residential | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total	10,108	13,635
Single family residential | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total	7,238	8,808
Single family residential | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	1,600
Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total	205,657	203,134
Commercial real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total	203,857	200,371
Commercial real estate | Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Total	1,800	2,763
Commercial real estate | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Commercial real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Commercial real estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Land, Construction and NM
Financing Receivable, Credit Quality Indicator [Line Items]
Total	22,957	20,542
Land, Construction and NM | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total	22,957	20,542
Land, Construction and NM | Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Land, Construction and NM | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Land, Construction and NM | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Land, Construction and NM | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total	$ 0	$ 0