Fair Value - Quantitative Information about Level 3 Fair Value Measurements for Financial Instruments Measured at Fair Value on Non Recurring Basis (Detail) - Sales Comparison Valuation Technique [Member] - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Impaired Loans - Commercial Mortgages [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 2,426	$ 2,353
Impaired Loans - Commercial, Industrial, and Agricultural [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 2,824	$ 2,820
Minimum [Member] | Impaired Loans - Commercial Mortgages [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Sales comparison approach	24.00%	34.00%
Minimum [Member] | Impaired Loans - Commercial, Industrial, and Agricultural [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Sales comparison approach		8.00%
Maximum [Member] | Impaired Loans - Commercial Mortgages [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Sales comparison approach	100.00%	100.00%
Maximum [Member] | Impaired Loans - Commercial, Industrial, and Agricultural [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Sales comparison approach	4.00%	49.00%
Weighted Average [Member] | Impaired Loans - Commercial Mortgages [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Sales comparison approach	(40.00%)	(44.00%)
Weighted Average [Member] | Impaired Loans - Commercial, Industrial, and Agricultural [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Sales comparison approach		(13.00%)