Fair Value Measurements and Disclosures - Assets Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	$ 282,318	$ 258,876
U.S. Treasuries [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	43,106	54,096
U.S. Government agencies [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	160,167	149,098
Mortgage-backed securities [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	41,865	17,441
States and political subdivisions [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	36,530	37,591
Equity securities [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	650	650
Level 1 [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities
Level 1 [Member] | U.S. Treasuries [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities
Level 1 [Member] | U.S. Government agencies [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities
Level 1 [Member] | Mortgage-backed securities [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities
Level 1 [Member] | States and political subdivisions [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities
Level 1 [Member] | Equity securities [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities
Level 2 [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	282,318	258,876
Level 2 [Member] | U.S. Treasuries [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	43,106	54,096
Level 2 [Member] | U.S. Government agencies [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	160,167	149,098
Level 2 [Member] | Mortgage-backed securities [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	41,865	17,441
Level 2 [Member] | States and political subdivisions [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	36,530	37,591
Level 2 [Member] | Equity securities [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	650	650
Level 3 [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities
Level 3 [Member] | U.S. Treasuries [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities
Level 3 [Member] | U.S. Government agencies [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities
Level 3 [Member] | Mortgage-backed securities [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities
Level 3 [Member] | States and political subdivisions [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities
Level 3 [Member] | Equity securities [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities