GOODWILL, OTHER INTANGIBLE ASSETS AND MORTGAGE SERVICING RIGHTS (Finite-Lived Intangible Assets) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Goodwill [Line Items]
Goodwill, beginning of period	$ 242,659	$ 244,583		$ 247,738
Intangible Assets, Net (Including Goodwill), beginning of period	265,314	274,745		285,210
Goodwill, Acquired During Period	96,495
Acquired Intangible Assets (Including Goodwill)	112,863
Amortization	(6,099)	(6,431)		(7,310)
Goodwill, Other Changes		(1,924)		(3,155)
Finite-Lived Intangible Assets, Other Changes		(1,076)	[1]	0
Intangible Assets (including Goodwill), Other Changes		(3,000)	[1]	(3,155)
Goodwill, end of period	339,154	242,659		244,583
Balance, end of period	32,699
Intangible Assets, Net (Including Goodwill), end of period	372,078	265,314		274,745
Core Deposit Intangibles [Member]
Goodwill [Line Items]
Balance, beginning of period	22,378	29,701		36,762
Additions through acquisition	16,368
Amortization	(6,047)	(6,247)		(7,061)
Balance, end of period	32,699	22,378		29,701
Other Intangible Assets [Member]
Goodwill [Line Items]
Balance, beginning of period	277	461		710
Amortization	(52)	(184)		(249)
Balance, end of period	$ 225	$ 277		$ 461

[1] 1) The adjustments to goodwill in 2016 related to changes in the preliminary goodwill recorded for the Starbuck Bancshares, Inc. acquisition including adjustments to loan discount, deferred taxes and REO valuations