Loans and Allowance for Loan Losses - Loans Outstanding by Impairment Methodology (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	$ 88,401	$ 76,758
Collectively evaluated for impairment	3,858,081	3,756,419
Receivables Acquired with Deteriorated Credit Quality [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans acquired with deteriorated credit quality	13,187	18,221
Non-residential Real Estate Owner Occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	6,846	5,374
Collectively evaluated for impairment	486,432	478,261
Non-residential Real Estate Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	34,900	25,251
Collectively evaluated for impairment	1,002,339	928,752
Residential Real Estate Permanent Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	7,181	7,313
Collectively evaluated for impairment	317,956	300,266
Residential Real Estate All Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	11,807	8,319
Collectively evaluated for impairment	651,228	628,769
Non-consumer Non-real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	25,302	28,156
Collectively evaluated for impairment	909,100	938,551
Consumer Non-real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	2,199	2,112
Collectively evaluated for impairment	266,169	245,653
Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	166	233
Collectively evaluated for impairment	161,160	152,751
Acquired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment	63,697	83,416
Acquired Loans [Member] | Receivables Acquired with Deteriorated Credit Quality [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans acquired with deteriorated credit quality	$ 13,187	$ 18,221