GOODWILL, OTHER INTANGIBLE ASSETS AND MORTGAGE SERVICING RIGHTS (Finite-Lived Intangible Assets) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Finite-lived Intangible Assets [Roll Forward]
Goodwill, beginning of period	$ 242,659	$ 244,583
Goodwill and other intangibles, net, beginning of period	265,314	274,745
Amortization	(4,156)	(6,431)
Goodwill, end of period	242,659	242,659
Goodwill and other intangibles, net, end of period	261,158	265,314
Goodwill, Other Increase (Decrease)		(1,924)
Intangible Assets (including Goodwill), Other Changes		(3,000)
Core Deposit Intangibles [Member]
Finite-lived Intangible Assets [Roll Forward]
Finite lived assets Balance, beginning of period	22,378	29,701
Amortization	(4,112)	(6,247)
Finite lived assets Balance, end of period	18,266	22,378
Finite-Lived Intangible Assets, Other Changes		(1,076)
Leasehold Improvements [Member]
Finite-lived Intangible Assets [Roll Forward]
Finite lived assets Balance, beginning of period	277	461
Amortization	(44)	(184)
Finite lived assets Balance, end of period	$ 233	277
Finite-Lived Intangible Assets, Other Changes		$ 0