Acquisition and Divestiture Activity - Additional Information (Detail) $ / shares in Units, $ in Thousands, shares in Millions					1 Months Ended	3 Months Ended			6 Months Ended		12 Months Ended
	Jan. 12, 2016 BankingCenters $ / shares	Aug. 14, 2015 USD ($) Branches	May 08, 2015 USD ($)	Feb. 01, 2015 USD ($)	May 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Mar. 30, 2016 USD ($)	Jun. 30, 2016 USD ($) shares	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) Branch	May 01, 2016 USD ($)	Jan. 01, 2015 Branch	Dec. 31, 2014 USD ($)
Business Acquisition [Line Items]
Goodwill acquired during the period									$ 111,852	$ 57,619
Purchasers assumed loans		$ 193,600
Other transaction expense related to divestitures		900
Deposits		$ 555,800
Goodwill allocation						$ 655,523			655,523	$ 588,464	$ 584,634			$ 530,845
Number of banking centers consolidated | Branch											23
Cash received in acquisition									91,771
Goodwill and intangible assets eliminated									41,000
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
Founders Financial Corporation [Member]
Business Acquisition [Line Items]
Number of branch locations | Branch													4
Total purchase price of acquisition						91,700			91,700
Purchase price of acquisition, cash									$ 41,000
Issuance of common stock shares for acquisitions of business | shares									3.4
Value of issuance common stock shares						50,600			$ 50,600
Transaction and integration costs associated with the acquisition									$ 4,900
Effective date of acquisition									Jan. 01, 2015
Goodwill allocation											$ 56,014
Founders Financial Corporation [Member] | Core Deposit [Member]
Business Acquisition [Line Items]
Estimated useful life of intangible assets									7 years
Estimated fair value of intangible assets acquired									$ 2,900
Mutual Underwriters [Member]
Business Acquisition [Line Items]
Total purchase price of acquisition				$ 3,700
Mutual Underwriters [Member] | Customer Business Relationships [Member]
Business Acquisition [Line Items]
Estimated useful life of intangible assets									10 years
Mutual Underwriters [Member] | Insurance [Member]
Business Acquisition [Line Items]
Goodwill acquired during the period				1,100
Mutual Underwriters [Member] | Insurance [Member] | Customer Business Relationships [Member]
Business Acquisition [Line Items]
Customer business relationship intangibles acquired				$ 2,600
Insurance Book of Business [Member] | Insurance [Member]
Business Acquisition [Line Items]
Purchase price of acquisition, cash			$ 100
Additional consideration upon operation targets			100
Insurance Book of Business [Member] | Insurance [Member] | Customer Business Relationships [Member]
Business Acquisition [Line Items]
Estimated useful life of intangible assets									10 years
Increase (decrease) in intangible assets			$ 200				$ (200)
Anchor BanCorp Wisconsin Inc [Member]
Business Acquisition [Line Items]
Total purchase price of acquisition						459,800			$ 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			$ 273,600
Transaction and integration costs associated with the acquisition								$ 7,200	$ 8,600
Acquisition agreement date									Jan. 12, 2016
Effective date of acquisition									May 01, 2016
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
Goodwill acquired during the period									$ 111,900
Goodwill allocation												$ 111,852
Anchor BanCorp Wisconsin Inc [Member] | Maximum [Member]
Business Acquisition [Line Items]
Maximum percentage of purchase price in cash	40.00%
Anchor BanCorp Wisconsin Inc [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
Increase (decrease) in intangible assets						$ 21,600
Anchor BanCorp Wisconsin Inc [Member] | Madison Milwaukee and Fox Valley Triangle [Member]
Business Acquisition [Line Items]
Number of banking centers | BankingCenters	32