LOANS - Aging Analysis (Details) $ in Thousands	Mar. 31, 2020 USD ($) loan	Dec. 31, 2019 USD ($)
Financing Receivable, Past Due [Line Items]
Non-accrual	$ 5,573	$ 6,333
Current	6,209,058	6,218,506
Ending balance	6,218,314	6,230,977
30 Days
Financing Receivable, Past Due [Line Items]
Past due	1,066	5,448
60 Days
Financing Receivable, Past Due [Line Items]
Past due	2,617	690
90 Days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Multifamily residential
Financing Receivable, Past Due [Line Items]
Non-accrual	537	541
Current	4,057,435	3,984,029
Ending balance	$ 4,058,869	3,985,981
Number of loans with COVID-related request | loan	1
COVID-related request, amount	$ 595
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	897	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	5,036	5,792
Current	1,923,009	2,010,801
Ending balance	$ 1,930,831	2,021,320
Number of loans with COVID-related request | loan	5
COVID-related request, amount	$ 6,000
Single family residential | 30 Days
Financing Receivable, Past Due [Line Items]
Past due	1,066	4,037
Single family residential | 60 Days
Financing Receivable, Past Due [Line Items]
Past due	1,720	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	205,657	203,134
Ending balance	205,657	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	22,957	20,542
Ending balance	22,957	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