GOODWILL, CORE DEPOSIT INTANGIBLES AND OTHER INTANGIBLE ASSETS (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 04, 2020	Dec. 31, 2019	Nov. 30, 2019	May 10, 2019
Goodwill [Line Items]
Goodwill resulting from acquisition	$ 576,005,000		$ 446,345,000
Goodwill, impairment loss	$ 0
AimBank
Goodwill [Line Items]
Goodwill resulting from acquisition		$ 91,400,000
AimBank | Core Deposit Intangibles
Goodwill [Line Items]
Recognized intangibles		3,100,000
Johnson Bank
Goodwill [Line Items]
Goodwill resulting from acquisition		38,400,000
Johnson Bank | Core Deposit Intangibles
Goodwill [Line Items]
Recognized intangibles		$ 1,300,000
Intangibles amortization period	10 years
Rockford Bank and Trust Company
Goodwill [Line Items]
Goodwill resulting from acquisition				$ 19,200,000
Rockford Bank and Trust Company | Core Deposit Intangibles
Goodwill [Line Items]
Recognized intangibles				$ 1,800,000
Intangibles amortization period	10 years
Blue Valley Ban Corp.
Goodwill [Line Items]
Goodwill resulting from acquisition					$ 35,400,000
Blue Valley Ban Corp. | Core Deposit Intangibles
Goodwill [Line Items]
Recognized intangibles					$ 11,400,000