GOODWILL, OTHER INTANGIBLE ASSETS AND MORTGAGE SERVICING RIGHTS (Finite-Lived Intangible Assets) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 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	(1,874)	(3,230)
Goodwill, end of period	244,811	247,738
Goodwill and other intangibles, net, end of period	280,409	285,210
Goodwill, Purchase Accounting Adjustments	(2,927)
Goodwill, Other Changes		0
Core Deposit Intangibles [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Other Changes		(300)
Finite-lived Intangible Assets [Roll Forward]
CDI Balance, beginning of period	36,762	2,831
Additions through acquisitions		37,395
Amortization	(1,808)	(3,164)
CDI Balance, end of period	34,954	36,762
Leasehold Improvements [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Other Changes		0
Finite-lived Intangible Assets [Roll Forward]
CDI Balance, beginning of period	710	0
Additions through acquisitions		776
Amortization	(66)	(66)
CDI Balance, end of period	$ 644	$ 710