Fair Values of Financial Instruments and Interest Rate Risk - Assets Measured at Fair Value on Nonrecurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 95,743	$ 105,899
Fair Value on a Nonrecurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	2,624	2,217
Other real estate owned	1,785	3,130
Fair Value	4,409	5,347
Total liabilities at fair value	0	0
Fair Value on a Nonrecurring Basis [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total liabilities at fair value	0	0
Fair Value on a Nonrecurring Basis [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total liabilities at fair value	0	0
Fair Value on a Nonrecurring Basis [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	2,624	2,217
Other real estate owned	1,785	3,130
Fair Value	4,409	5,347
Total liabilities at fair value	$ 0	$ 0