Fair Value Measurements and Disclosures - Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale, Equity securities	$ 458	$ 458	$ 650
Available for sale securities	245,664	233,578	202,807
U.S. Treasuries [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	122,644	147,624	63,754
U.S. Government Agencies [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	19,831	24,825	84,546
Mortgage-backed Securities [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	88,261	42,708	30,130
States and Political Subdivisions [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	14,470	17,963	23,727
Level 2 [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale, Equity securities	458	458	650
Available for sale securities	245,664	233,578	202,627
Level 2 [Member] | U.S. Treasuries [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	122,644	147,624	63,754
Level 2 [Member] | U.S. Government Agencies [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	19,831	24,825	84,546
Level 2 [Member] | Mortgage-backed Securities [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	88,261	42,708	30,130
Level 2 [Member] | States and Political Subdivisions [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities	$ 14,470	$ 17,963	23,547
Level 3 [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities			180
Level 3 [Member] | States and Political Subdivisions [Member]
Fair Value Assets Measured On Recurring Basis [Abstract]
Available for sale securities			$ 180