Short-Term Borrowings - Summary of Short-Term Borrowings (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Short-term borrowings	$ 900	$ 500
Weighted average interest rate	0.97%	0.38%
Federal Funds Purchased [Member]
Short-term Debt [Line Items]
Short-term borrowings	$ 900	$ 500
Short-term Debt [Member]
Short-term Debt [Line Items]
End of period interest rate	1.38%	0.63%