Acquisition and Divestiture Activity - Summary of Fair Values of Acquired Assets, Liabilities Assumed and Resulting Goodwill (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Nov. 01, 2018	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Goodwill	$ 1,036,258			$ 828,051	$ 655,018
Anchor Bank (MN) [Member] | Minnesota [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			33,433
Accrued interest receivable			5,872
Other real estate owned			1,058
Company-owned life insurance			44,490
Other assets			30,036
Deposits			(1,777,588)
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			132,374
Definite-lived intangible assets acquired			26,606
Goodwill			173,785
Total consideration			$ 332,765
KleinBank [Member]
Business Acquisition [Line Items]
Premises and equipment	$ 33,400
KleinBank [Member] | Minnesota [Member]
Business Acquisition [Line Items]
Cash and cash equivalents		$ 60,759
Investment securities		697,951
Federal Home Loan Bank stock		2,637
Loans held for sale		3,371
Loans		1,049,073
Premises and equipment		33,391
Accrued interest receivable		7,896
Other real estate owned		954
Company-owned life insurance		36,380
Net deferred tax assets		6,500
Other assets		10,299
Deposits		(1,713,086)
Securities sold under agreements to repurchase		(19,481)
Accrued expenses and other liabilities		(17,506)
Net tangible assets acquired		159,138
Definite-lived intangible assets acquired		39,017
Loan servicing rights		285
Goodwill		208,034
Total consideration		$ 406,474