GOODWILL, OTHER INTANGIBLE ASSETS AND MORTGAGE SERVICING RIGHTS (Finite-Lived Intangible Assets) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Goodwill [Roll Forward]
Goodwill, beginning of period	$ 373,121	$ 373,121
Goodwill, end of period	373,121	373,121
Finite-lived Intangible Assets [Roll Forward]
Goodwill and other intangibles, net, beginning of period	394,547	402,279
Amortization	(4,997)	(7,732)
Goodwill and other intangibles, net, end of period	389,550	394,547
Core Deposit Intangibles [Member]
Finite-lived Intangible Assets [Roll Forward]
Finite lived assets Balance, beginning of period	21,426	29,158
Amortization	(4,997)	(7,732)
Finite lived assets Balance, end of period	$ 16,429	$ 21,426