BUSINESS COMBINATIONS BUSINESS COMBINATIONS (Schedule of Acquisition Related Costs) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Business Acquisition [Line Items]
Acquisition related costs		$ 11,733	$ 26,110	$ 4,325
Six Oregon Branches [Member]
Business Acquisition [Line Items]
Acquisition related costs	$ 0	0		1,784
Siuslaw Financial Group, Inc [Member]
Business Acquisition [Line Items]
Acquisition related costs		95	2,000	748
Starbuck Bancshares, Inc [Member]
Business Acquisition [Line Items]
Acquisition related costs		11,638	24,110	1,793
Salary and Employee Benefits [Member]
Business Acquisition [Line Items]
Acquisition related costs		1,384	6,577	0
Occupancy and Equipment [Member]
Business Acquisition [Line Items]
Acquisition related costs		2,590	1,031	105
Advertising and Marketing [Member]
Business Acquisition [Line Items]
Acquisition related costs		1,158	527	327
Information and Computer Data Services [Member]
Business Acquisition [Line Items]
Acquisition related costs		2,490	2,875	334
Payment and Card Processing Expenses [Member]
Business Acquisition [Line Items]
Acquisition related costs		197	28	185
Professional Services [Member]
Business Acquisition [Line Items]
Acquisition related costs		2,230	11,169	2,953
Miscellaneous Other Noninterest Expense [Member]
Business Acquisition [Line Items]
Acquisition related costs		$ 1,684	$ 3,903	$ 421