Loans and the Allowance for Credit Losses - Recorded Investment in Non-Accrual Loans and TDRs (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Financing Receivable, Impaired [Line Items]
Non-accrual loans	$ 41,374	$ 38,818
TDRs	26,554	17,486
Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	17,330	12,066
Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	9,224	5,420
Commercial Real Estate - Land and Construction [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	1,463	1,488
TDRs	1,517	464
Commercial Real Estate - Land and Construction [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	1,013
Commercial Real Estate - Land and Construction [Member] | Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	504	464
Commercial Real Estate - Improved Property [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	22,143	20,227
TDRs	12,203	4,287
Commercial Real Estate - Improved Property [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	6,804	2,437
Commercial Real Estate - Improved Property [Member] | Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	5,399	1,850
Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	23,606	21,715
TDRs	13,720	4,751
Commercial Real Estate [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	7,817	2,437
Commercial Real Estate [Member] | Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	5,903	2,314
Commercial and Industrial [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	3,849	4,110
TDRs	727	807
Commercial and Industrial [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	351	329
Commercial and Industrial [Member] | Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	376	478
Residential Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	11,249	10,329
TDRs	10,269	10,289
Residential Real Estate [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	7,883	8,215
Residential Real Estate [Member] | Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	2,386	2,074
Home Equity [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	1,899	1,923
TDRs	1,003	985
Home Equity [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	718	740
Home Equity [Member] | Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	285	245
Consumer [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	771	741
TDRs	835	654
Consumer [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	561	345
Consumer [Member] | Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	$ 274	$ 309