LOANS - Aging Analysis (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Non-accrual	$ 4,891	$ 6,333
Current	6,276,111	6,218,506
Ending balance	6,281,039	6,230,977
30 Days
Financing Receivable, Past Due [Line Items]
Past due	0	5,448
60 Days
Financing Receivable, Past Due [Line Items]
Past due	37	690
90+ Days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Multifamily residential
Financing Receivable, Past Due [Line Items]
Non-accrual	533	541
Current	4,081,691	3,984,029
Ending balance	4,082,224	3,985,981
Multifamily residential | 30 Days
Financing Receivable, Past Due [Line Items]
Past due	0	1,411
Multifamily residential | 60 Days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Multifamily residential | 90+ Days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Single family residential
Financing Receivable, Past Due [Line Items]
Non-accrual	4,358	5,792
Current	1,965,168	2,010,801
Ending balance	1,969,563	2,021,320
Single family residential | 30 Days
Financing Receivable, Past Due [Line Items]
Past due	0	4,037
Single family residential | 60 Days
Financing Receivable, Past Due [Line Items]
Past due	37	690
Single family residential | 90+ Days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial real estate
Financing Receivable, Past Due [Line Items]
Non-accrual	0	0
Current	211,135	203,134
Ending balance	211,135	203,134
Commercial real estate | 30 Days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial real estate | 60 Days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial real estate | 90+ Days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Land, Construction and NM
Financing Receivable, Past Due [Line Items]
Non-accrual	0	0
Current	18,117	20,542
Ending balance	18,117	20,542
Land, Construction and NM | 30 Days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Land, Construction and NM | 60 Days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Land, Construction and NM | 90+ Days
Financing Receivable, Past Due [Line Items]
Past due	$ 0	$ 0