LOANS - Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance allocated to portfolio segments	$ 40,657	$ 36,001	$ 34,692	$ 36,001	$ 34,314
Provision for (reversal of) loan losses	5,250		450	10,550	750
Charge-offs	0		0	(722)	0
Recoveries	78		79	156	157
Ending balance allocated to portfolio segments	45,985	40,657	35,221	45,985	35,221
Ending allowance balance allocated to:
Loans individually evaluated for impairment						$ 25	$ 815
Loans collectively evaluated for impairment						45,960	35,186
Ending balance	40,657	36,001	35,221	36,001	35,221	45,985	36,001
Loans:
Ending balance: individually evaluated for impairment						6,176	7,638
Ending balance: collectively evaluated for impairment						6,274,863	6,223,339
Ending balance						6,281,039	6,230,977
Multifamily residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance allocated to portfolio segments	27,308	23,372	22,046	23,372	21,326
Provision for (reversal of) loan losses	3,529		699	7,465	1,419
Charge-offs	0		0	0	0
Recoveries	0		0	0	0
Ending balance allocated to portfolio segments	30,837	27,308	22,745	30,837	22,745
Ending allowance balance allocated to:
Loans individually evaluated for impairment						0	0
Loans collectively evaluated for impairment						30,837	23,372
Ending balance	30,837	23,372	22,745	23,372	22,745	30,837	23,372
Loans:
Ending balance: individually evaluated for impairment						533	541
Ending balance: collectively evaluated for impairment						4,081,691	3,985,440
Ending balance						4,082,224	3,985,981
Single family residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance allocated to portfolio segments	10,426	10,076	9,889	10,076	10,125
Provision for (reversal of) loan losses	1,199		(454)	2,268	(693)
Charge-offs	0		0	(722)	0
Recoveries	3		4	6	7
Ending balance allocated to portfolio segments	11,628	10,426	9,439	11,628	9,439
Ending allowance balance allocated to:
Loans individually evaluated for impairment						25	815
Loans collectively evaluated for impairment						11,603	9,261
Ending balance	11,628	10,076	9,439	10,076	9,439	11,628	10,076
Loans:
Ending balance: individually evaluated for impairment						5,643	7,097
Ending balance: collectively evaluated for impairment						1,963,920	2,014,223
Ending balance						1,969,563	2,021,320
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance allocated to portfolio segments	2,677	2,341	2,278	2,341	2,441
Provision for (reversal of) loan losses	628		134	964	(29)
Charge-offs	0		0	0	0
Recoveries	0		0	0	0
Ending balance allocated to portfolio segments	3,305	2,677	2,412	3,305	2,412
Ending allowance balance allocated to:
Loans individually evaluated for impairment						0	0
Loans collectively evaluated for impairment						3,305	2,341
Ending balance	3,305	2,341	2,412	3,305	2,412	3,305	2,341
Loans:
Ending balance: individually evaluated for impairment						0	0
Ending balance: collectively evaluated for impairment						211,135	203,134
Ending balance						211,135	203,134
Land, Construction and NM
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance allocated to portfolio segments	246	212	479	212	422
Provision for (reversal of) loan losses	(106)		71	(147)	53
Charge-offs	0		0	0	0
Recoveries	75		75	150	150
Ending balance allocated to portfolio segments	215	246	625	215	625
Ending allowance balance allocated to:
Loans individually evaluated for impairment						0	0
Loans collectively evaluated for impairment						215	212
Ending balance	$ 215	$ 212	$ 625	$ 215	$ 422	215	212
Loans:
Ending balance: individually evaluated for impairment						0	0
Ending balance: collectively evaluated for impairment						18,117	20,542
Ending balance						$ 18,117	$ 20,542