GOODWILL, OTHER INTANGIBLE ASSETS AND MORTGAGE SERVICING RIGHTS (Finite-Lived Intangible Assets) (Details) - USD ($) $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2016	Dec. 31, 2015
Finite-lived Intangible Assets [Roll Forward]
Goodwill, beginning of period		$ 247,738	$ 0
Goodwill and other intangibles, net, beginning of period		285,210	2,831
Goodwill, acquired during period			247,738
Acquired Intangible Assets (Including Goodwill)			285,909
Amortization		(5,538)	(3,230)
Goodwill, Purchase Accounting Adjustments		(3,155)
Goodwill, Other Changes			0
Goodwill, end of period		244,583	247,738
Goodwill and other intangibles, net, end of period		276,517	285,210
Core Deposit Intangibles [Member]
Finite-lived Intangible Assets [Roll Forward]
CDI Balance, beginning of period		36,762	2,831
Additions through acquisitions			37,395
Amortization		(5,339)	(3,164)
Finite-Lived Intangible Assets, Other Changes	[1]		(300)
CDI Balance, end of period		31,423	36,762
Leasehold Improvements [Member]
Finite-lived Intangible Assets [Roll Forward]
CDI Balance, beginning of period		710	0
Additions through acquisitions			776
Amortization		(199)	(66)
Finite-Lived Intangible Assets, Other Changes			0
CDI Balance, end of period		$ 511	$ 710

[1] (1) Acquired CDI from AmericanWest was adjusted for a branch that was subsequently sold.