Business Combinations (Narrative) (Detail) $ in Thousands						3 Months Ended	75 Months Ended
	Mar. 31, 2016 USD ($) locations	Jul. 24, 2015 USD ($) locations	Jul. 17, 2015 USD ($) locations	Jul. 02, 2015 USD ($)	Jan. 16, 2015 USD ($) locations	Mar. 31, 2016 USD ($) locations	Mar. 31, 2016 USD ($) locations financial_institution	Jul. 01, 2015 USD ($) locations
Business Acquisition [Line Items]
Goodwill, Acquired During Period						$ 11,305
FDIC loss sharing percentage on purchased loans, OREO, and certain other assets						80.00%
Generations Bancorp Inc.
Business Acquisition [Line Items]
Effective date of acquisition	Mar. 31, 2016
Number of locations | locations	1					1	1
Assets acquired	$ 134,200					$ 134,200	$ 134,200
Loans acquired	67,500					67,500	67,500
Assumed deposits	100,100					$ 100,100	$ 100,100
Goodwill, Acquired During Period	$ 11,300
Community Financial Shares, Inc.
Business Acquisition [Line Items]
Effective date of acquisition		Jul. 24, 2015
Number of locations | locations		4
Assets acquired		$ 350,500
Loans acquired		159,500
Assumed deposits		290,000
Goodwill, Acquired During Period		$ 27,600
Suburban Illinois Bancorp
Business Acquisition [Line Items]
Effective date of acquisition			Jul. 17, 2015
Number of locations | locations			10
Assets acquired			$ 494,700
Loans acquired			257,800
Assumed deposits			416,700
Goodwill, Acquired During Period			$ 18,600
North Bank
Business Acquisition [Line Items]
Effective date of acquisition				Jul. 01, 2015
Number of locations | locations								2
Assets acquired								$ 117,900
Loans acquired								51,600
Assumed deposits								$ 101,000
Goodwill, Acquired During Period				$ 6,700
Delavan Bancshares
Business Acquisition [Line Items]
Effective date of acquisition					Jan. 16, 2015
Number of locations | locations					4
Assets acquired					$ 224,100
Loans acquired					128,000
Assumed deposits					170,200
Goodwill, Acquired During Period					16,800
Liabilities assumed					$ 186,400
FDIC Assisted
Business Acquisition [Line Items]
Number of FDIC assisted banks acquired | financial_institution							9