Fair Value Measurements - Fair Value of Financial Assets and Liabilities Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale, at fair value	$ 491,022	$ 467,703
Derivative assets	331	295
Derivative liabilities	287	261
Loans held for sale	6,197	6,173
U.S. Treasuries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale, at fair value	341,249	312,802
U.S. Federal Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale, at fair value	85,900	88,851
Mortgage Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale, at fair value	17,685	17,976
States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale, at fair value	39,923	42,370
Other Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale, at fair value	6,265	5,704
Level 1 Inputs [Member] | U.S. Treasuries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale, at fair value	341,249	312,802
Level 2 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	331	295
Derivative liabilities	287	261
Loans held for sale	6,197	6,173
Level 2 Inputs [Member] | U.S. Federal Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale, at fair value	85,900	88,851
Level 2 Inputs [Member] | Mortgage Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale, at fair value	3,218	3,509
Level 2 Inputs [Member] | States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale, at fair value	39,923	42,370
Level 3 Inputs [Member] | Mortgage Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale, at fair value	14,467	14,467
Level 3 Inputs [Member] | Other Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale, at fair value	$ 6,265	$ 5,704