Intangible Asset - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill And Intangible Assets Disclosure [Abstract]
Intangible asset estimated weighted- average remaining useful life	12 years 7 months 6 days
Amortization of intangible asset	$ 646	$ 645	$ 646