Allowance for Loan Losses, Allowance for Losses on Lending-Related Commitments and Impaired Loans Allowance for Loan Losses, Allowance for Losses on Lending-Related Commitments and Impaired Loans (Narrative) (Detail)		3 Months Ended				9 Months Ended				12 Months Ended
		Sep. 30, 2015 USD ($) contracts		Sep. 30, 2014 USD ($) contracts		Sep. 30, 2015 USD ($) contracts		Sep. 30, 2014 USD ($) contracts		Sep. 30, 2015 USD ($) contracts		Sep. 30, 2014 USD ($) contracts		Dec. 31, 2014 USD ($)
Financing Receivable, Allowance for Credit Losses [Line Items]
TDRs, number | contracts		1	[1],[2]	3	[1],[2]	10	[1],[2]	16	[1],[2]	15	[3],[4]	21	[3],[4]
Allowance for loan losses related to impaired loans		$ 8,483,000		$ 6,577,000		$ 8,483,000		$ 6,577,000		$ 8,483,000		$ 6,577,000		$ 6,270,000
TDRs, Additions		$ 222,000	[1],[2]	$ 667,000	[1],[2]	$ 1,833,000	[1],[2]	$ 8,152,000	[1],[2]	5,648,000	[3],[4]	10,086,000	[3],[4]
Weighted average extension term		214 months		18 months		49 months		14 months
Weighted average stated interest rate, basis points		3.38%		2.61%		3.58%		1.78%
Weighted average interest only term						28 months		9 months
Loan forgiveness		$ 0		$ 0		$ 0		$ 0
Modification to Interest Only Payments
Financing Receivable, Allowance for Credit Losses [Line Items]
TDRs, number | contracts	[1]	0		0		2		4
TDRs, Additions	[1]	$ 0		$ 0		$ 219,000		$ 2,659,000
Financing Receivable
Financing Receivable, Allowance for Credit Losses [Line Items]
TDRs		59,320,000		83,385,000		$ 59,320,000		83,385,000		59,320,000		$ 83,385,000		$ 82,275,000
TDRs, number | contracts						114
Allowance for loan losses related to impaired loans		3,400,000				$ 3,400,000				3,400,000
Interest income, passage of time		98,000		$ 294,000		385,000		$ 529,000
Residential Real Estate | Financing Receivable
Financing Receivable, Allowance for Credit Losses [Line Items]
Foreclosed residential properties		$ 15,700,000				$ 15,700,000				$ 15,700,000

[1] Balances represent the recorded investment in the loan at the time of the restructuring.

[2] TDRs may have more than one modification representing a concession. As such, TDRs during the period may be represented in more than one of the categories noted above.

[3] Balances represent the recorded investment in the loan at the time of the restructuring.

[4] Total TDRs represent all loans restructured in TDRs during the previous twelve months from the date indicated.