Loans and Loans Held for Sale - Restructured Loans for Periods Presented (Detail) $ in Thousands	3 Months Ended				9 Months Ended				12 Months Ended
	Sep. 30, 2015 USD ($) SecurityLoan		Sep. 30, 2014 USD ($) SecurityLoan		Sep. 30, 2015 USD ($) SecurityLoan		Sep. 30, 2014 USD ($) SecurityLoan		Dec. 31, 2014 USD ($)
Financing Receivable, Modifications [Line Items]
Threshold period of satisfactory performance for TDR to be restored to accruing status					6 months
Restructured loans	$ 1,578	[1]	$ 1,179	[1]	$ 5,353	[2]	$ 3,198	[2]
Number of TDRs returned to accruing status | SecurityLoan	3		5		9		10
TDRs returned to accruing status	$ 200		$ 500		$ 500		$ 2,000
Performing Financial Instruments [Member]
Financing Receivable, Modifications [Line Items]
Restructured loans					26,049				$ 36,983
Performing Financial Instruments [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Restructured loans					8,062				16,939
Performing Financial Instruments [Member] | Commercial Construction [Member]
Financing Receivable, Modifications [Line Items]
Restructured loans					5,627				5,736
Performing Financial Instruments [Member] | Residential Mortgage [Member]
Financing Receivable, Modifications [Line Items]
Restructured loans					2,609				2,839
Performing Financial Instruments [Member] | Home Equity [Member]
Financing Receivable, Modifications [Line Items]
Restructured loans					3,363				3,342
Performing Financial Instruments [Member] | Installment and Other Consumer [Member]
Financing Receivable, Modifications [Line Items]
Restructured loans					28				53
Nonperforming Financial Instruments [Member]
Financing Receivable, Modifications [Line Items]
Restructured loans					8,092				5,436
Nonperforming Financial Instruments [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Restructured loans					3,552				2,180
Nonperforming Financial Instruments [Member] | Commercial Construction [Member]
Financing Receivable, Modifications [Line Items]
Restructured loans					1,610				1,869
Nonperforming Financial Instruments [Member] | Residential Mortgage [Member]
Financing Receivable, Modifications [Line Items]
Restructured loans					591				459
Nonperforming Financial Instruments [Member] | Home Equity [Member]
Financing Receivable, Modifications [Line Items]
Restructured loans					412				562
Nonperforming Financial Instruments [Member] | Installment and Other Consumer [Member]
Financing Receivable, Modifications [Line Items]
Restructured loans					88				10
Financing Receivable [Member]
Financing Receivable, Modifications [Line Items]
Restructured loans					34,141				42,419
Financing Receivable [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Restructured loans					11,614				19,119
Financing Receivable [Member] | Commercial Construction [Member]
Financing Receivable, Modifications [Line Items]
Restructured loans					7,237				7,605
Financing Receivable [Member] | Residential Mortgage [Member]
Financing Receivable, Modifications [Line Items]
Restructured loans					3,200				3,298
Financing Receivable [Member] | Home Equity [Member]
Financing Receivable, Modifications [Line Items]
Restructured loans					3,775				3,904
Financing Receivable [Member] | Installment and Other Consumer [Member]
Financing Receivable, Modifications [Line Items]
Restructured loans					116				63
Commercial and Industrial [Member] | Performing Financial Instruments [Member]
Financing Receivable, Modifications [Line Items]
Restructured loans					6,360				8,074
Commercial and Industrial [Member] | Nonperforming Financial Instruments [Member]
Financing Receivable, Modifications [Line Items]
Restructured loans					1,839				356
Commercial and Industrial [Member] | Financing Receivable [Member]
Financing Receivable, Modifications [Line Items]
Restructured loans					$ 8,199				$ 8,430

[1] Excludes loans that were fully paid off or fully charged-off by period end. The pre-modification balance represents the balance outstanding prior to modification. The post-modification balance represents the outstanding balance at period end.

[2] Excludes loans that were fully paid off or fully charged-off by period end. The pre-modification balance represents the balance outstanding prior to modification. The post-modification balance represents the outstanding balance at period end.