Goodwill and Intangible Assets - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Goodwill And Intangible Assets Disclosure [Abstract]
Goodwill	$ 27,095,000		$ 27,095,000		$ 27,095,000
Change in carrying amount of goodwill			0		$ 0
Amortization of core deposit intangible assets	158,000	$ 172,000	483,000	$ 527,000
Aggregate mortgage servicing rights amortization	$ 23,000	$ 30,000	$ 51,000	$ 45,000