Acquisitions - Bank Mutual Acquisition (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Feb. 01, 2018	Dec. 31, 2017	Sep. 30, 2017
Business Acquisition [Line Items]
Goodwill	$ 1,168,922		$ 976,239
Total liabilities	$ 2,340,294			$ 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		2,930
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)		199
Others assets		7,054
Deposits		2,498
Other borrowings		1,875
Other liabilities		4,487
February 1, 2018
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		42,689
Bank owned life insurance		65,390
Goodwill		175,499
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)		4,848
Others assets		47,158
Total assets		2,820,506
Deposits		1,840,950
Other borrowings		431,886
Other liabilities		65,982
Total liabilities		2,338,818
Total consideration paid		$ 481,688