LOANS AND CREDIT QUALITY - Aging Analysis (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Nonaccrual	$ 21,541	$ 20,722
Total past due and nonaccrual	34,231	35,140
Current	5,257,543	5,209,040
Total	$ 5,291,774	$ 5,244,180
Nonaccrual, percent of total loans	0.41%	0.40%
Past due and nonaccrual, percent of total loans	0.65%	0.67%
Not past due, percent of total loans	99.35%	99.33%
Percent of total loans	100.00%	100.00%
Federal Housing Administration, Veteran Affairs, Or Small Business Administration
Financing Receivable, Past Due [Line Items]
Total past due and nonaccrual	$ 13,500	$ 14,700
Recurring
Financing Receivable, Past Due [Line Items]
Fair value of loans held for investment	4,324	7,108
Level 3 | Recurring
Financing Receivable, Past Due [Line Items]
Fair value of loans held for investment	4,324	7,108
30-59 days
Financing Receivable, Past Due [Line Items]
Past due	$ 858	$ 2,389
Past due, percent of total loans	0.02%	0.05%
60-89 days
Financing Receivable, Past Due [Line Items]
Past due	$ 1,156	$ 553
Past due, percent of total loans	0.02%	0.01%
90+ days
Financing Receivable, Past Due [Line Items]
Past due	$ 10,676	$ 11,476
Past due, percent of total loans	0.20%	0.22%
Commercial loans
Financing Receivable, Past Due [Line Items]
Nonaccrual	$ 14,137	$ 14,105
Total	4,050,057
Commercial loans | Real Estate Sector
Financing Receivable, Past Due [Line Items]
Total	2,819,553	2,811,325
Commercial loans | Commercial and Industrial Sector
Financing Receivable, Past Due [Line Items]
Nonaccrual	14,137	14,105
Total past due and nonaccrual	14,137	14,105
Current	1,216,367	1,098,874
Total	1,230,504	1,112,979
Commercial loans | Commercial and Industrial Sector | 30-59 days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial loans | Commercial and Industrial Sector | 60-89 days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial loans | Commercial and Industrial Sector | 90+ days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial loans | Non-owner occupied commercial real estate
Financing Receivable, Past Due [Line Items]
Nonaccrual	0	0
Total past due and nonaccrual	0	0
Current	766,002	829,538
Total	766,002	829,538
Commercial loans | Non-owner occupied commercial real estate | 30-59 days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial loans | Non-owner occupied commercial real estate | 60-89 days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial loans | Non-owner occupied commercial real estate | 90+ days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial loans | Multifamily
Financing Receivable, Past Due [Line Items]
Nonaccrual	0	0
Total past due and nonaccrual	0	0
Current	1,521,349	1,428,092
Total	1,521,349	1,428,092
Commercial loans | Multifamily | 30-59 days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial loans | Multifamily | 60-89 days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial loans | Multifamily | 90+ days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial loans | Multifamily construction
Financing Receivable, Past Due [Line Items]
Nonaccrual	0
Total past due and nonaccrual	0
Current	106,780	115,329
Total	106,780	115,329
Commercial loans | Multifamily construction | 30-59 days
Financing Receivable, Past Due [Line Items]
Past due	0
Commercial loans | Multifamily construction | 60-89 days
Financing Receivable, Past Due [Line Items]
Past due	0
Commercial loans | Multifamily construction | 90+ days
Financing Receivable, Past Due [Line Items]
Past due	0
Commercial loans | Commercial real estate construction
Financing Receivable, Past Due [Line Items]
Nonaccrual	0	0
Total past due and nonaccrual	0	0
Current	28,610	27,285
Total	28,610	27,285
Commercial loans | Commercial real estate construction | 30-59 days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial loans | Commercial real estate construction | 60-89 days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial loans | Commercial real estate construction | 90+ days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial loans | Single family construction
Financing Receivable, Past Due [Line Items]
Nonaccrual	0	0
Total past due and nonaccrual	0	0
Current	269,501	259,170
Total	269,501	259,170
Commercial loans | Single family construction | 30-59 days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial loans | Single family construction | 60-89 days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial loans | Single family construction | 90+ days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial loans | Single family construction to permanent
Financing Receivable, Past Due [Line Items]
Nonaccrual	0	0
Total past due and nonaccrual	0	0
Current	127,311	151,911
Total	127,311	151,911
Commercial loans | Single family construction to permanent | 30-59 days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Total	0
Commercial loans | Single family construction to permanent | 60-89 days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Total	0
Commercial loans | Single family construction to permanent | 90+ days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Total	0
Commercial loans | Owner occupied commercial real estate
Financing Receivable, Past Due [Line Items]
Nonaccrual	4,913	4,922
Total past due and nonaccrual	4,913	4,922
Current	468,360	462,334
Total	473,273	467,256
Commercial loans | Owner occupied commercial real estate | 30-59 days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial loans | Owner occupied commercial real estate | 60-89 days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial loans | Owner occupied commercial real estate | 90+ days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial loans | Commercial business
Financing Receivable, Past Due [Line Items]
Nonaccrual	9,224	9,183
Total past due and nonaccrual	9,224	9,183
Current	748,007	636,540
Total	757,231	645,723
Commercial loans | Commercial business | 30-59 days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial loans | Commercial business | 60-89 days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial loans | Commercial business | 90+ days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Consumer loans
Financing Receivable, Past Due [Line Items]
Nonaccrual	7,404	6,617
Total	1,241,717	1,319,876
Consumer loans | Single family
Financing Receivable, Past Due [Line Items]
Nonaccrual	5,583	4,883
Total past due and nonaccrual	18,065	18,938
Current	857,352	896,185
Total	875,417	915,123
Consumer loans | Single family | 30-59 days
Financing Receivable, Past Due [Line Items]
Past due	653	2,161
Total	532
Consumer loans | Single family | 60-89 days
Financing Receivable, Past Due [Line Items]
Past due	1,153	418
Total	644
Consumer loans | Single family | 90+ days
Financing Receivable, Past Due [Line Items]
Past due	10,676	11,476
Total	3,886
Consumer loans | Home equity, adjusted for credit analysis
Financing Receivable, Past Due [Line Items]
Nonaccrual	1,821	1,734
Total past due and nonaccrual	2,029	2,097
Current	364,271	402,656
Total	366,300	404,753
Consumer loans | Home equity, adjusted for credit analysis | 30-59 days
Financing Receivable, Past Due [Line Items]
Past due	205	228
Consumer loans | Home equity, adjusted for credit analysis | 60-89 days
Financing Receivable, Past Due [Line Items]
Past due	3	135
Consumer loans | Home equity, adjusted for credit analysis | 90+ days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial loans, adjusted for credit analysis | Real Estate Sector
Financing Receivable, Past Due [Line Items]
Nonaccrual	0	0
Total past due and nonaccrual	0	0
Current	2,819,553	2,811,325
Total	2,819,553	2,811,325
Commercial loans, adjusted for credit analysis | Real Estate Sector | 30-59 days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial loans, adjusted for credit analysis | Real Estate Sector | 60-89 days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial loans, adjusted for credit analysis | Real Estate Sector | 90+ days
Financing Receivable, Past Due [Line Items]
Past due	0	0
Consumer loans, adjusted for credit analysis
Financing Receivable, Past Due [Line Items]
Nonaccrual	7,404	6,617
Total past due and nonaccrual	20,094	21,035
Current	1,221,623	1,298,841
Total	1,241,717	1,319,876
Consumer loans, adjusted for credit analysis | 30-59 days
Financing Receivable, Past Due [Line Items]
Past due	858	2,389
Consumer loans, adjusted for credit analysis | 60-89 days
Financing Receivable, Past Due [Line Items]
Past due	1,156	553
Consumer loans, adjusted for credit analysis | 90+ days
Financing Receivable, Past Due [Line Items]
Past due	10,676	$ 11,476
Consumer loans, adjusted for credit analysis | Home equity, adjusted for credit analysis
Financing Receivable, Past Due [Line Items]
Total	366,300
Consumer loans, adjusted for credit analysis | Home equity, adjusted for credit analysis | 30-59 days
Financing Receivable, Past Due [Line Items]
Total	212
Consumer loans, adjusted for credit analysis | Home equity, adjusted for credit analysis | 60-89 days
Financing Receivable, Past Due [Line Items]
Total	12
Consumer loans, adjusted for credit analysis | Home equity, adjusted for credit analysis | 90+ days
Financing Receivable, Past Due [Line Items]
Total	$ 1,474