Acquisitions - Bank Mutual Acquisition (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Feb. 01, 2018	Dec. 31, 2017	Jun. 30, 2017
Business Acquisition [Line Items]
Goodwill	$ 1,166,665		$ 976,239
Total liabilities	$ 2,828,448			$ 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		(48,043)
Premises and equipment, net		5,180
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 (included in other assets on the face of the Consolidated Balance Sheet)		989
Others assets		6,153
Deposits		2,498
Other borrowings		1,875
Other liabilities		4,370
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		1,875,877
Premises and equipment, net		44,939
Bank owned life insurance		65,390
Goodwill		173,243
Core deposit intangibles (included in other intangible assets, net on the face of the Consolidated Balance Sheet)		58,100
Other real estate owned (included in other assets on the face of the Consolidated Balance Sheet)		5,638
Others assets		46,257
Total assets		2,820,389
Deposits		1,840,950
Other borrowings		431,886
Other liabilities		65,864
Total liabilities		2,338,701
Total consideration paid		$ 481,688