Securities - Summary of amortized cost and fair value by contractual maturity of securities available for sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale securities, Amortized Cost	$ 83,775	$ 77,773
Total available-for-sale securities, Fair Value	81,215	77,153
State and Municipal Obligations
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	978	3,583
Due after one year but less than five years, Amortized Cost	45,633	37,747
Due after five years but less than ten years, Amortized Cost	28,063	28,441
Due after ten years, Amortized Cost	9,101	8,002
Total available-for-sale securities, Amortized Cost	20,525	21,113
Due in one year or less, Fair Value	973	3,514
Due after one year but less than five years, Fair Value	44,382	37,425
Due after five years but less than ten years, Fair Value	27,236	28,250
Due after ten years, Fair Value	8,624	7,964
Total available-for-sale securities, Fair Value	$ 19,938	$ 21,153