Goodwill and Intangible Assets (Details) - USD ($)		3 Months Ended		12 Months Ended
	Feb. 01, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Goodwill, Impairment Loss				$ 0	$ 0	$ 0
Goodwill				$ 1,169,023,000	$ 976,239,000
Bank Mutual [Member]
Business Acquisition [Line Items]
Goodwill acquired during period	$ 175,000,000
Additions to finite-lived intangible assets	$ 58,000,000
Diversified Insurance [Member]
Business Acquisition [Line Items]
Goodwill acquired during period			$ 10,000,000
Additions to finite-lived intangible assets			$ 8,000,000
Anderson Insurance Investment Agency [Domain]
Business Acquisition [Line Items]
Goodwill acquired during period		$ 7,000,000
Additions to finite-lived intangible assets		$ 3,000,000