Qualified Affordable Housing Project Investments - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Investments In Affordable Housing Projects [Abstract]
Investment for qualified affordable housing projects included in other assets	$ 3,630,000		$ 3,204,000
Unfunded commitments related to the investments in qualified affordable housing projects	3,955,000		$ 4,510,000
Recognized amortization expense	129,000	$ 82,000
Recognized tax credits and other benefits from its investment in affordable housing tax credits	219,000	138,000
Impairment losses related to its investment in qualified affordable housing projects	$ 0	$ 0