BUSINESS COMBINATIONS BUSINESS COMBINATIONS (Textual) (Details) $ / shares in Units, shares in Thousands, $ in Thousands	Oct. 01, 2015 USD ($) bank_branch shares	Mar. 06, 2015 USD ($) $ / shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Business Acquisition [Line Items]
Assets	$ 9,900,000		$ 9,793,668	$ 9,796,298
Number of Branches | bank_branch	203
Goodwill			$ 244,583	$ 247,738	$ 0	$ 0
Starbuck Bancshares, Inc [Member]
Business Acquisition [Line Items]
Effective date of acquisition	Oct. 01, 2015
Cash paid	$ 130,000
Business Combination, Consideration Transferred, Equity Interests Issued and Issuable, Shares | shares	13,230
Business Combination, Acquired Assets	$ 4,459,985
Business Combination, Acquired Deposits	3,638,596
Business Combination, Acquired Receivables	2,999,130
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	33,500
Goodwill	$ 222,892
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	10 years
Business Combination, Acquired Receivables, Non-Credit-Impaired	$ 2,950,000
Business Combination, Acquired Receivables, Non-Credit-Impaired, Fair Value	2,940,000
Business Combination, Acquired Receivables, Discount	17,700
Business Combination, Acquired Receivables, Non-Credit-Impaired, Cash Flows Not Expected to be Collected at Acquisition	44,100
Property and equipment	66,728
Cash and cash equivalents	$ 95,821
Siuslaw Financial Group, Inc [Member]
Business Acquisition [Line Items]
Effective date of acquisition		Mar. 06, 2015
Cash paid		$ 5,806
Business Combination, Acquired Assets		369,763
Business Combination, Acquired Deposits		316,406
Business Combination, Acquired Receivables		247,098
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles		3,895
Goodwill		$ 21,691
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life		8 years
Business Combination, Acquired Receivables, Non-Credit-Impaired		$ 244,200
Business Combination, Acquired Receivables, Non-Credit-Impaired, Fair Value		241,400
Business Combination, Acquired Receivables, Discount		$ 2,800
Business Acquisition, Equity Interest Issued to Acquiree's Shareholders, Per Share | $ / shares		$ 0.32231
Business Acquisition, Cash Payment Issued to Acquiree's Shareholders, Per Share | $ / shares		$ 1.41622
Property and equipment		$ 8,127
Cash and cash equivalents		$ 84,405
Washington, Oregon, Idaho, California and Utah [Member] | Starbuck Bancshares, Inc [Member]
Business Acquisition [Line Items]
Business Combination, Number of Branches | bank_branch	98