Business Combinations (Narrative) (Detail)									9 Months Ended	69 Months Ended
	Jul. 24, 2015 USD ($) locations	Jul. 17, 2015 USD ($) locations	Jul. 02, 2015 USD ($)	Jan. 16, 2015 USD ($) locations	Aug. 08, 2014 USD ($) locations	Jul. 11, 2014 USD ($)	May. 16, 2014 USD ($)	Apr. 28, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2015 financial_institution	Jul. 01, 2015 USD ($) locations
Business Acquisition [Line Items]
Goodwill, Acquired During Period									$ 69,398,000
FDIC loss sharing percentage on purchased loans, OREO, and certain other assets									80.00%
Community Financial Shares, Inc.
Business Acquisition [Line Items]
Effective date of acquisition	Jul. 24, 2015
Number of locations | locations	4
Assets acquired	$ 350,500,000
Loans acquired	159,500,000
Assumed deposits	290,000,000
Goodwill, Acquired During Period	$ 27,500,000
Suburban Illinois Bancorp
Business Acquisition [Line Items]
Effective date of acquisition		Jul. 17, 2015
Number of locations | locations		10
Assets acquired		$ 494,700,000
Loans acquired		257,800,000
Assumed deposits		416,700,000
Goodwill, Acquired During Period		$ 18,800,000
North Bank
Business Acquisition [Line Items]
Effective date of acquisition			Jul. 01, 2015
Number of locations | locations											2
Assets acquired											$ 101,000,000
Loans acquired											51,600,000
Assumed deposits											$ 100,300,000
Goodwill, Acquired During Period			$ 6,700,000
Delavan Bancshares
Business Acquisition [Line Items]
Effective date of acquisition				Jan. 16, 2015
Number of locations | locations				4
Assets acquired				$ 224,100,000
Loans acquired				128,000,000
Assumed deposits				170,200,000
Liabilities assumed				186,400,000
Goodwill, Acquired During Period				$ 16,300,000
Talmer Bank & Trust
Business Acquisition [Line Items]
Effective date of acquisition					Aug. 08, 2014
Number of locations | locations					11
Assets acquired					$ 361,300,000
Loans acquired					41,500,000
Assumed deposits					354,900,000
Liabilities assumed					361,300,000
Goodwill, Acquired During Period					$ 9,700,000
THE National Bank
Business Acquisition [Line Items]
Effective date of acquisition						Jul. 11, 2014
Assets acquired						$ 94,100,000
Loans acquired						75,000,000
Assumed deposits						36,200,000
Goodwill, Acquired During Period						$ 16,300,000
Urban Partnership Bank, Stone Park branch
Business Acquisition [Line Items]
Effective date of acquisition							May 16, 2014
Assumed deposits							$ 5,400,000
Liabilities assumed							5,500,000
Goodwill, Acquired During Period							$ 678,000
FDIC Assisted
Business Acquisition [Line Items]
Number of FDIC assisted banks acquired | financial_institution										9
Policy Billing Services Inc. And Equity Premium Finance Inc.
Business Acquisition [Line Items]
Effective date of acquisition								Apr. 28, 2014
Loans acquired								$ 7,400,000
Goodwill, Acquired During Period								$ 6,500,000
Number of affiliated companies acquired								2