BUSINESS COMBINATIONS BUSINESS COMBINATIONS (Schedule of Components of Acquisition) (Details) - USD ($) $ in Thousands	Oct. 01, 2015	Mar. 06, 2015	Sep. 30, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair value of liabilities assumed:
Goodwill			$ 244,583	$ 247,738	$ 0
Siuslaw Financial Group, Inc [Member]
Business Acquisition [Line Items]
Acquisition Date		Mar. 06, 2015
Consideration transferred:
Cash paid		$ 5,806
Fair value of common shares issued		58,100
Total consideration		63,906
Fair value of assets acquired:
Cash and cash equivalents		84,405
Securities—available-for-sale		12,865
Loans receivable		247,098
REO, held for sale		2,525
Property and equipment		8,127
Core deposit intangible		3,895
Other assets		10,848
Total assets acquired		369,763
Fair value of liabilities assumed:
Deposits		316,406
Junior subordinated debentures		5,959
Other liabilities		5,183
Total liabilities assumed		327,548
Net assets acquired		42,215
Goodwill		21,691
Business Combination, Acquired Receivables, Gross Contractual Amount		$ 252,200
Starbuck Bancshares, Inc [Member]
Business Acquisition [Line Items]
Acquisition Date	Oct. 01, 2015
Consideration transferred:
Cash paid	$ 130,000
Fair value of common shares issued	630,674
Total consideration	760,674
Fair value of assets acquired:
Cash and cash equivalents	95,821
Securities—available-for-sale	1,037,238
Loans receivable	2,999,130
REO, held for sale	6,105
Property and equipment	66,728
Core deposit intangible	33,500
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Assets Noncurrent	108,454
Other assets	113,009
Total assets acquired	4,459,985
Fair value of liabilities assumed:
Deposits	3,638,596
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Financial Liabilities	221,442
Junior subordinated debentures	5,806
Other liabilities	56,359
Total liabilities assumed	3,922,203
Net assets acquired	537,782
Goodwill	222,892
Business Combination, Acquired Receivables, Gross Contractual Amount	$ 3,040,000