LOANS - Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance allocated to portfolio segments	$ 36,001	$ 34,314
Provision for (reversal of) loan losses	5,300	300
Charge-offs	(722)	0
Recoveries	78	78
Ending balance allocated to portfolio segments	40,657	34,692
Ending allowance balance allocated to:
Loans individually evaluated for impairment			$ 25	$ 815
Loans collectively evaluated for impairment			40,632	35,186
Ending balance	40,657	34,692	40,657	36,001
Loans:
Ending balance: individually evaluated for impairment			6,868	7,638
Ending balance: collectively evaluated for impairment			6,211,446	6,223,339
Ending balance			6,218,314	6,230,977
Multifamily residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance allocated to portfolio segments	23,372	21,326
Provision for (reversal of) loan losses	3,936	720
Charge-offs	0	0
Recoveries	0	0
Ending balance allocated to portfolio segments	27,308	22,046
Ending allowance balance allocated to:
Loans individually evaluated for impairment			0	0
Loans collectively evaluated for impairment			27,308	23,372
Ending balance	27,308	22,046	27,308	23,372
Loans:
Ending balance: individually evaluated for impairment			537	541
Ending balance: collectively evaluated for impairment			4,058,332	3,985,440
Ending balance			4,058,869	3,985,981
Single family residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance allocated to portfolio segments	10,076	10,125
Provision for (reversal of) loan losses	1,069	(239)
Charge-offs	(722)	0
Recoveries	3	3
Ending balance allocated to portfolio segments	10,426	9,889
Ending allowance balance allocated to:
Loans individually evaluated for impairment			25	815
Loans collectively evaluated for impairment			10,401	9,261
Ending balance	10,426	9,889	10,426	10,076
Loans:
Ending balance: individually evaluated for impairment			6,331	7,097
Ending balance: collectively evaluated for impairment			1,924,500	2,014,223
Ending balance			1,930,831	2,021,320
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance allocated to portfolio segments	2,341	2,441
Provision for (reversal of) loan losses	336	(163)
Charge-offs	0	0
Recoveries	0	0
Ending balance allocated to portfolio segments	2,677	2,278
Ending allowance balance allocated to:
Loans individually evaluated for impairment			0	0
Loans collectively evaluated for impairment			2,677	2,341
Ending balance	2,677	2,278	2,677	2,341
Loans:
Ending balance: individually evaluated for impairment			0	0
Ending balance: collectively evaluated for impairment			205,657	203,134
Ending balance			205,657	203,134
Land, Construction and NM
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance allocated to portfolio segments	212	422
Provision for (reversal of) loan losses	(41)	(18)
Charge-offs	0	0
Recoveries	75	75
Ending balance allocated to portfolio segments	246	479
Ending allowance balance allocated to:
Loans individually evaluated for impairment			0	0
Loans collectively evaluated for impairment			246	212
Ending balance	$ 246	$ 479	246	212
Loans:
Ending balance: individually evaluated for impairment			0	0
Ending balance: collectively evaluated for impairment			22,957	20,542
Ending balance			$ 22,957	$ 20,542