Acquisition and Divestiture Activity - Schedule of Consideration Paid Allocation (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Deposits	$ (10,683,714)	$ (10,743,253)	$ (10,451,602)
Securities sold under agreements to repurchase	(298,094)	(367,052)	(354,123)
Goodwill	655,018	$ 655,018	$ 655,523	$ 584,634
Anchor BanCorp Wisconsin Inc [Member]
Business Acquisition [Line Items]
Cash and cash equivalents	123,657
Investment securities	235,240
Federal Home Loan Bank 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 paid	$ 459,754