Fair Value - Assets and Liabilities Measured at Fair Value on a Non-Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Commercial Mortgages [Member]
Impaired loans:
Fair value assets and liabilities measured on a nonrecurring basis	$ 7,474	$ 8,422
Commercial, Industrial, and Agricultural [Member]
Impaired loans:
Fair value assets and liabilities measured on a nonrecurring basis	1,200	1,973
Residential Real Estate [Member]
Impaired loans:
Fair value assets and liabilities measured on a nonrecurring basis	64	402
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Commercial Mortgages [Member]
Impaired loans:
Fair value assets and liabilities measured on a nonrecurring basis	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Commercial, Industrial, and Agricultural [Member]
Impaired loans:
Fair value assets and liabilities measured on a nonrecurring basis	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Residential Real Estate [Member]
Impaired loans:
Fair value assets and liabilities measured on a nonrecurring basis	0	0
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgages [Member]
Impaired loans:
Fair value assets and liabilities measured on a nonrecurring basis	0	0
Significant Other Observable Inputs (Level 2) [Member] | Commercial, Industrial, and Agricultural [Member]
Impaired loans:
Fair value assets and liabilities measured on a nonrecurring basis	0	0
Significant Other Observable Inputs (Level 2) [Member] | Residential Real Estate [Member]
Impaired loans:
Fair value assets and liabilities measured on a nonrecurring basis	0	0
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgages [Member]
Impaired loans:
Fair value assets and liabilities measured on a nonrecurring basis	7,474	8,422
Significant Unobservable Inputs (Level 3) [Member] | Commercial, Industrial, and Agricultural [Member]
Impaired loans:
Fair value assets and liabilities measured on a nonrecurring basis	1,200	1,973
Significant Unobservable Inputs (Level 3) [Member] | Residential Real Estate [Member]
Impaired loans:
Fair value assets and liabilities measured on a nonrecurring basis	$ 64	$ 402