Fair Value Measurements of Assets Recognized in Consolidated Balance Sheets Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
U.S. government agencies
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair value of asset, recurring basis	$ 44,430	$ 44,959
State and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair value of asset, recurring basis	42,036
Fair Value, Inputs, Level 1 | U.S. government agencies
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair value of asset, recurring basis	0	0
Fair Value, Inputs, Level 1 | State and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair value of asset, recurring basis	0
Fair Value, Inputs, Level 2 | U.S. government agencies
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair value of asset, recurring basis	44,430	44,959
Fair Value, Inputs, Level 2 | State and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair value of asset, recurring basis	42,036
Fair Value, Inputs, Level 3 | U.S. government agencies
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair value of asset, recurring basis	0	$ 0
Fair Value, Inputs, Level 3 | State and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair value of asset, recurring basis	$ 0