Fair Value Measurements and Disclosures - Assets Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale, Equity securities	$ 650	$ 650	$ 650
Available for sale securities	215,122	275,440	258,876
U.S. Treasuries [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	29,654	43,648	54,096
U.S. Government agencies [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	117,989	145,805	149,098
Mortgage-backed securities [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	35,817	50,326	17,441
States and political subdivisions [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	31,012	35,011	37,591
Level 2 [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale, Equity securities	650	650	650
Available for sale securities	215,122	275,440	258,876
Level 2 [Member] | U.S. Treasuries [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	29,654	43,648	54,096
Level 2 [Member] | U.S. Government agencies [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	117,989	145,805	149,098
Level 2 [Member] | Mortgage-backed securities [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	35,817	50,326	17,441
Level 2 [Member] | States and political subdivisions [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	$ 31,012	$ 35,011	$ 37,591