Goodwill and Other Intangible Assets (Details Textuals) - USD ($)			3 Months Ended	6 Months Ended
	Jun. 01, 2018	Feb. 01, 2018	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill			$ 1,166,665,000	$ 1,166,665,000		$ 976,239,000
Goodwill, Impairment Loss				0	$ 0
Bank Mutual
Business Acquisition [Line Items]
Goodwill acquired		$ 173,000,000		173,000,000
Additions to finite-lived intangible assets			58,000,000
Diversified Insurance Solutions
Business Acquisition [Line Items]
Goodwill acquired				$ 10,000,000
Additions to finite-lived intangible assets		$ 8,000,000
Anderson Insurance Investment Agency [Domain]
Business Acquisition [Line Items]
Goodwill acquired	$ 7,000,000		7,000,000
Additions to finite-lived intangible assets	$ 3,000,000		$ 3,000,000