Goodwill and Other Intangible Assets (Details Textuals) - USD ($)		3 Months Ended		9 Months Ended
	Feb. 01, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill				$ 1,168,922,000		$ 976,239,000
Goodwill, Impairment Loss				$ 0	$ 0
Bank Mutual
Business Acquisition [Line Items]
Goodwill acquired	$ 175,000,000
Approximate additions to finite-lived intangible assets	$ 58,000,000
Diversified Insurance Solutions
Business Acquisition [Line Items]
Goodwill acquired			$ 10,000,000
Approximate additions to finite-lived intangible assets			$ 8,000,000
Anderson Insurance Investment Agency
Business Acquisition [Line Items]
Goodwill acquired		$ 7,000,000
Approximate additions to finite-lived intangible assets		$ 3,000,000