Subsequent Events Subsequent Events (Narrative) (Detail) - Subsequent Event [Member] $ in Millions	Jul. 24, 2015 USD ($) locations	Jul. 17, 2015 USD ($) locations	Jul. 02, 2015	Jul. 01, 2015 USD ($) locations
Community Financial Shares, Inc.
Subsequent Event [Line Items]
Business Acquisition, Effective Date of Acquisition	Jul. 24, 2015
Number of locations | locations	4
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	$ 327
Business Combination, Acquired Receivables, Fair Value	177
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deposits	301
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Junior Subordinated Debentures	$ 4
Suburban Illinois Bancorp
Subsequent Event [Line Items]
Business Acquisition, Effective Date of Acquisition		Jul. 17, 2015
Number of locations | locations		10
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets		$ 480
Business Combination, Acquired Receivables, Fair Value		284
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deposits		417
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Junior Subordinated Debentures		$ 15
North Bank
Subsequent Event [Line Items]
Business Acquisition, Effective Date of Acquisition			Jul. 01, 2015
Number of locations | locations				2
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets				$ 112
Business Combination, Acquired Receivables, Fair Value				55
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deposits				$ 100