Securities - Additional Information (Detail) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2015	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Securities pledged for public funds	$ 141,600,000			$ 141,600,000		$ 129,800,000
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)
Proceeds from the sale of securities available for sale	0	$ 9,700,000		10,361,000	$ 62,906,000
Gross gains realized on sales of securities available for sale	0	236,000		233,000	610,000
Gross losses realized on sales of securities available for sale	$ 0	$ 0		$ 0	$ 44,000