Securities - Additional Information (Detail) - USD ($)	3 Months Ended			6 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2015	Jun. 30, 2019	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Held to maturity securities transferred to available for sale, amortized cost				$ 74,600,000
Held to maturity securities transferred to available for sale, fair value				$ 72,500,000
Securities pledged for public funds	$ 166,500,000				$ 166,500,000		$ 108,700,000
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	5,100,000	$ 0			63,400,000	$ 10,400,000
Gross gains realized on sales of securities available for sale	128,000	0			452,000	233,000
Gross losses realized on sales of securities available for sale	$ 0	$ 0			$ 32,000	$ 0