Business Combinations (Narrative) (Detail) (USD $)	3 Months Ended	0 Months Ended				63 Months Ended	0 Months Ended
	Mar. 31, 2015	Jan. 16, 2015 locations	Aug. 08, 2014	Jul. 11, 2014	May 16, 2014	Mar. 31, 2015 financial_institution	Apr. 28, 2014
Business Acquisition [Line Items]
Additional goodwill recorded on acquisition	$ 16,718,000
FDIC loss sharing percentage on purchased loans, OREO, and certain other assets	80.00%
Delavan Bancshares
Business Acquisition [Line Items]
Effective date of acquisition		Jan. 16, 2015
Number of locations		4
Assets acquired		223,900,000
Loans acquired		128,000,000
Liabilities assumed		185,600,000
Assumed deposits		170,200,000
Additional goodwill recorded on acquisition		16,700,000
Talmer Bank & Trust
Business Acquisition [Line Items]
Effective date of acquisition			Aug. 08, 2014
Number of locations			11
Assets acquired			361,300,000
Loans acquired			41,500,000
Liabilities assumed			361,300,000
Assumed deposits			354,900,000
Additional goodwill recorded on acquisition			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
Additional goodwill recorded on acquisition				16,300,000
Urban Partnership Bank, Stone Park branch
Business Acquisition [Line Items]
Effective date of acquisition					May 16, 2014
Liabilities assumed					5,500,000
Assumed deposits					5,400,000
Additional goodwill recorded on acquisition					678,000
FDIC Assisted
Business Acquisition [Line Items]
Number of FDIC assisted banks acquired						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
Additional goodwill recorded on acquisition							$ 6,500,000
Number of affiliated companies acquired							2