Fair Value Measurements and Disclosures - Assets Measured at Fair Value on Non-Recurring Basis of Impairment Loans (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value measured on non-recurring basis, impaired loans	$ 6,511	$ 5,006	$ 4,981
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value measured on non-recurring basis, impaired loans	$ 6,511	$ 5,006	$ 4,981