Qualified Affordable Housing Project Investments - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Schedule Of Equity Method Investments [Line Items]
Investment in qualified affordable housing projects	$ 5,600,000		$ 5,600,000		$ 5,800,000
Unfunded commitments of investments in qualified affordable housing projects	4,500,000		4,500,000		$ 5,300,000
Recognized amortization expense included in income tax expense	122,000	$ 36,000	243,000	$ 90,000
Recognized tax credits and other benefits from investment in affordable housing tax credits	138,000	44,000	277,000	110,000
Impairment losses related to investment in qualified affordable housing projects	$ 0	$ 0	$ 0	$ 0
Minimum [Member]
Schedule Of Equity Method Investments [Line Items]
Expected period for fulfillment of investment commitments			8 years
Maximum [Member]
Schedule Of Equity Method Investments [Line Items]
Expected period for fulfillment of investment commitments			10 years