Securities Sold Under Agreement to Repurchase and Other Borrowings - Summary of Balances and Interest Rates (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Securities Sold Under Agreements To Repurchase [Abstract]
Average daily balance during the year	$ 3,584,000	$ 523,000	$ 29,891,000
Average interest rate during the year	0.59%	4.00%	4.19%
Maximum month end balance during the year	$ 12,643,000	$ 535,000	$ 30,556,000
Weighted average interest rate at year end	0.25%	4.00%	4.00%