STOCKHOLDERS' EQUITY (Details) - USD ($)				12 Months Ended
	Apr. 28, 2017	Apr. 29, 2016	Apr. 01, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock Transaction [Line Items]
Cumulative amount repurchased				$ 46,300,000
Cumulative shares repurchased (in shares)				1,100,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount				$ 7,700,000
Common stock issued in acquisitions				0	$ 62,246,000	$ 165,295,000
Net cash received in business combinations				0	$ 9,119,000	$ 66,517,000
Financial Advisor
Stock Transaction [Line Items]
Common stock issued in acquisitions			$ 2,600,000
Merger Agreement | As recorded by Baylake Corp
Stock Transaction [Line Items]
Number of common stock issued for consideration		4,344,243
Common stock issued in acquisitions		$ 163,300,000
Additional consideration for assumed stock options		1,200,000
Costs incurred related to stock issuance		$ 300,000
Merger Agreement | First Menasha
Stock Transaction [Line Items]
Number of common stock issued for consideration	1,309,885
Common stock issued in acquisitions	$ 62,200,000
Net cash received in business combinations	19,300,000
Costs incurred related to stock issuance	$ 200,000
Common stock repurchase program
Stock Transaction [Line Items]
Value of shares authorized to repurchased				$ 54,000,000
Shares authorized to repurchased				1,450,000
Stock repurchased under plan				$ 22,200,000
Number of shares cancelled under plan (in shares)				408,100
Weighted average price of share cancelled				$ 54.35