Business Combinations (Narrative) (Detail) $ in Thousands							9 Months Ended
	Aug. 19, 2016 USD ($)	Mar. 31, 2016 USD ($) locations	Jul. 24, 2015 USD ($) locations	Jul. 17, 2015 USD ($) locations	Jul. 02, 2015 USD ($)	Jan. 16, 2015 USD ($) locations	Sep. 30, 2016 USD ($) financial_institution	Jul. 01, 2015 USD ($) locations
Business Acquisition [Line Items]
Goodwill recorded on acquisition							$ 11,470
FDIC loss sharing percentage on purchased loans, OREO, and certain other assets							80.00%
GE Capital Franchise Finance
Business Acquisition [Line Items]
Effective date of acquisition	Aug. 19, 2016
Loans acquired	$ 560,900
Generations Bancorp Inc.
Business Acquisition [Line Items]
Effective date of acquisition		Mar. 31, 2016
Loans acquired		$ 67,400
Assets acquired		$ 131,000
Number of locations | locations		1
Assumed deposits		$ 100,200
Goodwill recorded on acquisition		$ 11,500
Community Financial Shares, Inc.
Business Acquisition [Line Items]
Effective date of acquisition			Jul. 24, 2015
Loans acquired			$ 159,500
Assets acquired			$ 350,500
Number of locations | locations			4
Assumed deposits			$ 290,000
Goodwill recorded on acquisition			$ 27,600
Suburban Illinois Bancorp
Business Acquisition [Line Items]
Effective date of acquisition				Jul. 17, 2015
Loans acquired				$ 257,800
Assets acquired				$ 494,700
Number of locations | locations				10
Assumed deposits				$ 416,700
Goodwill recorded on acquisition				$ 18,600
North Bank
Business Acquisition [Line Items]
Effective date of acquisition					Jul. 01, 2015
Loans acquired								$ 51,600
Assets acquired								$ 117,900
Number of locations | locations								2
Assumed deposits								$ 101,000
Goodwill recorded on acquisition					$ 6,700
Delavan Bancshares
Business Acquisition [Line Items]
Effective date of acquisition						Jan. 16, 2015
Loans acquired						$ 128,000
Assets acquired						$ 224,100
Number of locations | locations						4
Assumed deposits						$ 170,200
Goodwill recorded on acquisition						16,800
Liabilities assumed						$ 186,400
FDIC Assisted
Business Acquisition [Line Items]
Number of FDIC assisted banks acquired | financial_institution							9