Fair Value - Assets and Liabilities Measured at Fair Value on Non-Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Commercial Mortgages [Member]
Impaired loans:
Fair value assets and liabilities measured on a nonrecurring basis	$ 2,426	$ 2,353
Commercial, Industrial, and Agricultural [Member]
Impaired loans:
Fair value assets and liabilities measured on a nonrecurring basis	2,824	2,820
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
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 Unobservable Inputs (Level 3) [Member] | Commercial Mortgages [Member]
Impaired loans:
Fair value assets and liabilities measured on a nonrecurring basis	2,426	2,353
Significant Unobservable Inputs (Level 3) [Member] | Commercial, Industrial, and Agricultural [Member]
Impaired loans:
Fair value assets and liabilities measured on a nonrecurring basis	$ 2,824	$ 2,820