BUSINESS COMBINATION AND BRANCH DIVESTITURE Statement (Details) $ in Thousands, shares in Millions	Nov. 01, 2018 USD ($) bank_branch shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield		$ 5,216	$ 6,520	$ 8,717
Goodwill		$ 339,154	$ 242,659	$ 244,583	$ 247,738
Skagit Bank [Member]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents	$ 329
Business Acquisition, Effective Date of Acquisition	Nov. 01, 2018
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, Cash Flows Expected to be Collected at Acquisition	$ 7,982
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield	(995)
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value	$ 6,987
Business combination, number of branches | bank_branch	11
Business Combination, Consideration Transfered, Equity Interests Issued and Issuable, Shares | shares	3.1
Business Combination, Consideration Transfered, Equity Interests Issued and Issuable, Shares	$ 179,709
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	915,821
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deposits	810,209
Business Combination, Acquired Receivable, Fair Value	632,374
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	16,368
Goodwill	96,495
Business Combination, Acquired Receivables, Non-Credit-Impaired	637,400
Business Combination, consideration transfered	180,038
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents	19,167
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Marketable Securities	210,326
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Real Estate Held for Sale	2,593
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment	15,788
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Liabilities Noncurrent	95
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Assets	19,110
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liability, Other	22,069
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	832,278
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	83,543
Business Combination, Acquired Receivables, Non-Credit-Impaired, Fair Value	625,200
Business Combination, Acquired Receivables, Non-Credit-Impaired, Discount	12,200
Business Combination, Acquired Receivables, Non-Credit-Impaired, Credit Related Discount	7,900
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, Contractually Required Payments Receivable at Acquisition	9,897
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, Non-Accretable Difference	$ (1,915)