Securities - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Income tax expense related to net realized gains and losses		$ 198,000	$ 211,000	$ 50,000
Securities pledged for public funds		129,800,000	146,500,000
Net unrealized loss, net of taxes, on securities at date of transfer			(245,000)
Remaining unaccreted unrealized holding loss		$ 1,035,000	$ 1,230,000
Mortgage-Backed GSE Securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Available for sale securities transferred to held to maturity, total amortized cost	$ 105,300,000
Available for sale securities transferred to held to maturity fair value	103,800,000
Net unrealized loss, net of taxes, on securities at date of transfer	$ (999,000)