Acquisitions - Bank Mutual Acquisition (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Feb. 01, 2018	Dec. 31, 2017	Mar. 31, 2017
Business Acquisition [Line Items]
Goodwill	$ 1,153,156		$ 976,239
Total liabilities	$ 2,809,565			$ 0
Purchase Accounting Adjustments
Business Acquisition [Line Items]
Cash and cash equivalents		$ 0
Investment securities		(6,238)
Federal Home Loan Bank stock, at cost		0
Loans		(48,533)
Premises and equipment, net		11,172
Bank owned life insurance		(24)
Goodwill
Core deposit intangibles (included in other intangible assets, net on the face of the Consolidated Balance Sheet)		58,100
Other real estate owned		(246)
Others assets		5,098
Deposits		2,498
Other borrowings		935
Other liabilities		2,148
BKMU Assets Acquired, Liabilities Assumed
Business Acquisition [Line Items]
Cash and cash equivalents		78,052
Investment securities		452,867
Federal Home Loan Bank stock, at cost		20,026
Loans		1,875,386
Premises and equipment, net		50,930
Bank owned life insurance		65,390
Goodwill		166,870
Core deposit intangibles (included in other intangible assets, net on the face of the Consolidated Balance Sheet)		58,100
Other real estate owned		4,403
Others assets		37,151
Total assets		2,809,175
Deposits		1,840,950
Other borrowings		430,946
Other liabilities		55,591
Total liabilities		2,327,487
Total consideration paid		$ 481,688