Qualified Affordable Housing Project Investments - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Investments In Affordable Housing Projects [Abstract]
Investment for qualified affordable housing projects included in other assets	$ 3,079,000		$ 3,079,000		$ 2,754,000
Unfunded commitments related to the investments in qualified affordable housing projects	4,741,000		4,741,000		$ 2,313,000
Recognized amortization expense	82,000	$ 77,000	247,000	$ 231,000
Recognized tax credits and other benefits from its investment in affordable housing tax credits	138,000	147,000	414,000	442,000
Impairment losses related to its investment in qualified affordable housing projects	$ 0	$ 0	$ 0	$ 0