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LOANS AND CREDIT QUALITY - Loans Held for Investment (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Loans held for investment
Total LHFI	$ 5,354,257		$ 5,244,180
Allowance for credit losses	(54,516)	$ (59,897)	(64,294)	$ (64,892)	$ (65,000)	$ (41,772)
Loans held for investment	5,299,741		5,179,886
Recurring
Loans held for investment
Fair value of loans held for investment	4,507		7,108
Recurring \| Level 3
Loans held for investment
Fair value of loans held for investment	4,507		7,108
Commercial loans
Loans held for investment
Total LHFI	4,244,615		3,924,304
Commercial loans \| Real Estate Sector
Loans held for investment
Total LHFI	3,358,509		2,811,325
Allowance for credit losses	(23,841)	(25,544)	(27,935)	(28,136)	(24,347)	(22,535)
Commercial loans \| Commercial and Industrial Sector
Loans held for investment
Total LHFI	886,106		1,112,979
Allowance for credit losses	(19,758)	(21,392)	(22,037)	(24,032)	(21,457)	(6,554)
Commercial loans \| Non-owner occupied commercial real estate
Loans held for investment
Total LHFI	754,031		829,538
Allowance for credit losses	(9,636)	(9,077)	(8,845)	(8,923)	(7,325)	(7,245)
Commercial loans \| Multifamily
Loans held for investment
Total LHFI	2,090,156		1,428,092
Allowance for credit losses	(5,457)	(7,245)	(6,072)	(4,871)	(5,387)	(7,015)
Commercial loans \| Construction/land development
Loans held for investment
Total LHFI	514,322		553,695
Commercial loans \| Owner occupied commercial real estate
Loans held for investment
Total LHFI	450,350		467,256
Allowance for credit losses	(5,285)	(5,518)	(4,994)	(5,688)	(5,641)	(3,639)
Commercial loans \| Commercial business
Loans held for investment
Total LHFI	435,756		645,723
Allowance for credit losses	(14,473)	(15,874)	(17,043)	(18,344)	(15,816)	(2,915)
Consumer loans
Loans held for investment
Total LHFI	1,109,642		1,319,876
Allowance for credit losses	(10,917)	(12,961)	(14,322)	(12,724)	(19,196)	(12,683)
Consumer loans \| Single family
Loans held for investment
Total LHFI	793,927		915,123
Allowance for credit losses	(5,757)	(7,163)	(6,906)	(6,720)	(8,070)	(6,450)
Consumer loans \| Home equity and other
Loans held for investment
Total LHFI	315,715		404,753
Allowance for credit losses	$ (5,160)	$ (5,798)	$ (7,416)	$ (6,004)	$ (11,126)	$ (6,233)