LOANS - Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance allocated to portfolio segments	$ 45,985	$ 35,221	$ 36,001	$ 34,314
Provision for (reversal of) loan losses	0	(500)	10,550	250
Charge-offs	0	0	(722)	0
Recoveries	78	202	234	359
Ending balance allocated to portfolio segments	46,063	34,923	46,063	34,923
Ending allowance balance allocated to:
Loans individually evaluated for impairment					$ 25	$ 815
Loans collectively evaluated for impairment					46,038	35,186
Ending balance	46,063	34,923	46,063	34,314	46,063	36,001
Loans:
Ending balance: individually evaluated for impairment					6,103	7,638
Ending balance: collectively evaluated for impairment					6,142,398	6,223,339
Ending balance					6,148,501	6,230,977
Multifamily residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance allocated to portfolio segments	30,837	22,745	23,372	21,326
Provision for (reversal of) loan losses	836	408	8,301	1,827
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Ending balance allocated to portfolio segments	31,673	23,153	31,673	23,153
Ending allowance balance allocated to:
Loans individually evaluated for impairment					0	0
Loans collectively evaluated for impairment					31,673	23,372
Ending balance	31,673	23,153	23,372	21,326	31,673	23,372
Loans:
Ending balance: individually evaluated for impairment					528	541
Ending balance: collectively evaluated for impairment					4,085,531	3,985,440
Ending balance					4,086,059	3,985,981
Single family residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance allocated to portfolio segments	11,628	9,439	10,076	10,125
Provision for (reversal of) loan losses	(1,014)	(415)	1,254	(1,108)
Charge-offs	0	0	(722)	0
Recoveries	3	2	9	9
Ending balance allocated to portfolio segments	10,617	9,026	10,617	9,026
Ending allowance balance allocated to:
Loans individually evaluated for impairment					25	815
Loans collectively evaluated for impairment					10,592	9,261
Ending balance	10,617	9,026	10,076	9,026	10,617	10,076
Loans:
Ending balance: individually evaluated for impairment					5,575	7,097
Ending balance: collectively evaluated for impairment					1,833,581	2,014,223
Ending balance					1,839,156	2,021,320
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance allocated to portfolio segments	3,305	2,412	2,341	2,441
Provision for (reversal of) loan losses	247	115	1,211	86
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Ending balance allocated to portfolio segments	3,552	2,527	3,552	2,527
Ending allowance balance allocated to:
Loans individually evaluated for impairment					0	0
Loans collectively evaluated for impairment					3,552	2,341
Ending balance	3,552	2,527	2,341	2,527	3,552	2,341
Loans:
Ending balance: individually evaluated for impairment					0	0
Ending balance: collectively evaluated for impairment					203,920	203,134
Ending balance					203,920	203,134
Land, Construction and NM
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance allocated to portfolio segments	215	625	212	422
Provision for (reversal of) loan losses	(69)	(608)	(216)	(555)
Charge-offs	0	0	0	0
Recoveries	75	200	225	350
Ending balance allocated to portfolio segments	221	217	221	217
Ending allowance balance allocated to:
Loans individually evaluated for impairment					0	0
Loans collectively evaluated for impairment					221	212
Ending balance	$ 221	$ 217	$ 221	$ 217	221	212
Loans:
Ending balance: individually evaluated for impairment					0	0
Ending balance: collectively evaluated for impairment					19,366	20,542
Ending balance					$ 19,366	$ 20,542