BUSINESS COMBINATIONS BUSINESS COMBINATIONS (Textual) (Details) $ / shares in Units, shares in Thousands, $ in Thousands			3 Months Ended
	Oct. 01, 2015 USD ($) bank_branch shares	Mar. 06, 2015 USD ($) $ / shares	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Business Acquisition [Line Items]
Goodwill			$ 244,811	$ 247,738	$ 0
Assets	$ 9,900,000		$ 9,745,594	$ 9,796,298
Number of Stores | bank_branch	203
Starbuck Bancshares, Inc [Member]
Business Acquisition [Line Items]
Acquisition Date	Oct. 01, 2015
Cash paid in acquisition	$ 130,000
Common Stock Issued, Shares | shares	13,230
Fair value of common shares issued	$ 630,674
Business Combination, Consideration Transferred	760,674
Total assets acquired	4,459,758
Loans receivable acquired	2,999,130
Goodwill	223,119
Core deposit intangible	33,500
Deposits acquired	$ 3,638,596
Acquired finite-lived intangible assets, weighted average useful life	10 years
Acquired receivables, non-credit-impaired	$ 2,950,000
Acquired receivables, non-credit-impaired, fair value	2,940,000
Acquired receivables, discount	(17,700)
Business Combination, Acquired Receivables, Non-Credit-Impaired, Cash Flows Not Expected to be Collected at Acquisition	$ 44,100
Siuslaw Financial Group, Inc [Member]
Business Acquisition [Line Items]
Acquisition Date		Mar. 06, 2015
Cash paid in acquisition		$ 5,806
Fair value of common shares issued		$ 58,100
Equity interest issued to Siuslaw's shareholders per share | $ / shares		$ 0.32231
Cash payment issued to Siuslaw's shareholders (usd per share) | $ / shares		$ 1.41622
Business Combination, Consideration Transferred		$ 63,906
Total assets acquired		369,763
Loans receivable acquired		247,098
Goodwill		21,691
Core deposit intangible		3,895
Deposits acquired		316,406
Acquired finite-lived intangible assets, weighted average useful life			8 years
Acquired receivables, non-credit-impaired		244,200
Acquired receivables, non-credit-impaired, fair value		241,400
Acquired receivables, discount		$ (2,800)
Washington, Oregon, Idaho, California and Utah [Member] | Starbuck Bancshares, Inc [Member]
Business Acquisition [Line Items]
Number of branches acquired | bank_branch	98