Acquisition and Divestiture Activity - Additional Information (Detail) $ / shares in Units, $ in Thousands, shares in Millions			1 Months Ended	12 Months Ended
	May 01, 2016 BankingCenters	Aug. 14, 2015 USD ($) Branches	May 31, 2016 USD ($)	Dec. 31, 2017 USD ($) Branch $ / shares shares	Dec. 31, 2016 USD ($) Branch	Dec. 31, 2015 USD ($) Branch	Nov. 01, 2017 Branch
Business Acquisition [Line Items]
Purchasers assumed loans		$ 193,600
Other transaction expense related to divestitures		900
Deposits		$ 555,800
Net pre-tax gain						$ 15,627
Goodwill allocation				$ 828,051	$ 655,018	$ 584,634
Number of banking centers consolidated | Branch				5	5	23
Cash received in acquisition					$ 91,771
Goodwill and intangible assets eliminated					$ 40,963
ONB Insurance Group, Inc. [Member]
Business Acquisition [Line Items]
Cash received in acquisition			$ 91,800
Gain on sale of assets before tax			41,900
Gain on sale of assets after tax			17,600
Goodwill and intangible assets eliminated			$ 47,500
Southern Illinois [Member]
Business Acquisition [Line Items]
Number of banking centers | Branches		12
Eastern Indiana [Member]
Business Acquisition [Line Items]
Number of banking centers | Branches		4
Ohio [Member]
Business Acquisition [Line Items]
Number of banking centers | Branches		1
Southern Illinois Franchise [Member]
Business Acquisition [Line Items]
Net pre-tax gain		$ 15,600
Deposit premium		19,300
Goodwill allocation		$ 3,800
AnchorBank WI [Member]
Business Acquisition [Line Items]
Number of banking centers | BankingCenters	46
Portion of share received by merged entity				3.5505
Consideration received in cash by merged entity | $ / shares				$ 48.50
Total fair value of consideration paid				$ 459,800
Purchase price of acquisition, cash				$ 186,200
Issuance of common stock shares for acquisitions of business | shares				20.4
Value of issuance common stock shares				$ 273,600
Acquisition related costs incurred				15,900
Goodwill allocation				$ 111,347
AnchorBank WI [Member] | Maximum [Member]
Business Acquisition [Line Items]
Maximum percentage of purchase price in cash				40.00%
AnchorBank WI [Member] | Core Deposit [Member]
Business Acquisition [Line Items]
Estimated useful life of intangible assets				7 years
Estimated fair value of intangible assets acquired				$ 21,600
AnchorBank WI [Member] | Madison Milwaukee and Fox Valley Triangle [Member]
Business Acquisition [Line Items]
Number of banking centers | BankingCenters	32
Anchor Bank (MN) [Member]
Business Acquisition [Line Items]
Portion of share received by merged entity				1.350
Consideration received in cash by merged entity | $ / shares				$ 2.625
Total fair value of consideration paid				$ 332,800
Purchase price of acquisition, cash				$ 31,900
Issuance of common stock shares for acquisitions of business | shares				300.8
Value of issuance common stock shares				$ 300,800
Acquisition related costs incurred				12,300
Goodwill allocation				$ 173,033
Anchor Bank (MN) [Member] | Core Deposit [Member]
Business Acquisition [Line Items]
Estimated useful life of intangible assets				10 years
Estimated fair value of intangible assets acquired				$ 27,300
Anchor Bank (MN) [Member] | Minnesota [Member]
Business Acquisition [Line Items]
Number of branches | Branch							17