Acquisition and Divestiture Activity - Summary of Fair Values of Acquired Assets, Liabilities Assumed and Resulting Goodwill (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Goodwill	$ 828,051	$ 655,018	$ 584,634
AnchorBank WI [Member]
Business Acquisition [Line Items]
Cash and cash equivalents	123,657
Investment securities	235,240
FHLB stock	4,596
Loans held for sale	9,334
Loans	1,637,806
Premises and equipment	35,721
Accrued interest receivable	7,308
Other real estate owned	17,349
Company-owned life insurance	7,278
Other assets	126,210
Deposits	(1,852,713)
Securities sold under agreements to repurchase	(3,132)
Other borrowings	(123)
Accrued expenses and other liabilities	(36,957)
Net tangible assets acquired	311,574
Definite-lived intangible assets acquired	21,559
Loan servicing rights	15,274
Goodwill	111,347
Total consideration	459,754
Anchor Bank (MN) [Member]
Business Acquisition [Line Items]
Cash and cash equivalents	34,501
Investment securities	302,195
FHLB/Federal Reserve Bank stock	6,585
Loans held for sale	1,407
Loans	1,593,991
Premises and equipment	33,556
Accrued interest receivable	5,872
Other real estate owned	1,058
Company-owned life insurance	44,490
Other assets	29,872
Deposits	(1,777,449)
Federal funds purchased and interbank borrowings	(45,600)
Securities sold under agreements to repurchase	(22,965)
Other borrowings	(49,257)
Accrued expenses and other liabilities	(25,784)
Net tangible assets acquired	132,472
Definite-lived intangible assets acquired	27,260
Goodwill	173,033
Total consideration	$ 332,765