GOODWILL, OTHER INTANGIBLE ASSETS AND MORTGAGE SERVICING RIGHTS (Finite-Lived Intangible Assets) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Goodwill [Roll Forward]
Goodwill, beginning of period	$ 373,121	$ 339,154
Goodwill, Acquired During Period		33,967
Goodwill, end of period	373,121	373,121
Finite-lived Intangible Assets [Roll Forward]
Goodwill and other intangibles, net, beginning of period	402,279	372,078
Additions through acquisitions		38,577
Amortization	(5,867)	(8,151)
LHI, Other Increase (Decrease)		(225)
Goodwill and other intangibles, net, end of period	396,412	402,279
Finite-Lived Intangible Assets, adjustments, total		(225)
Core Deposit Intangibles [Member]
Finite-lived Intangible Assets [Roll Forward]
Finite lived assets Balance, beginning of period	29,158	32,699
Intangible assets, additions through acquisitions		4,610
Amortization	(5,867)	(8,151)
Finite lived assets Balance, end of period	23,291	29,158
Leasehold Improvements [Member]
Finite-lived Intangible Assets [Roll Forward]
Finite lived assets Balance, beginning of period	0	225
Intangible assets, additions through acquisitions		0
Amortization	0	0
Finite lived assets Balance, end of period	$ 0	$ 0