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GOODWILL, CORE DEPOSIT PREMIUM AND OTHER INTANGIBLE ASSETS (Goodwill narrative) (Details) - USD ($)		3 Months Ended
	Dec. 04, 2020	Jun. 30, 2020	Mar. 31, 2021	Dec. 31, 2020
Goodwill [Line Items]
Goodwill			$ 576,005,000	$ 576,005,000
Goodwill impairment		$ 0
AIM Bancshares, Inc.
Goodwill [Line Items]
Goodwill	$ 91,400,000
AIM Bancshares, Inc. \| Core Deposits
Goodwill [Line Items]
Intangibles recognized	$ 3,100,000
Intangibles, amortization period	10 years
Johnson Bank Branches
Goodwill [Line Items]
Goodwill	$ 38,400,000
Johnson Bank Branches \| Core Deposits
Goodwill [Line Items]
Intangibles recognized	$ 1,300,000
Intangibles, amortization period	10 years