Intangible Assets - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Amortization expense for intangible assets	$ 648	$ 475
Estimated aggregate amortization expense of intangible assets in 2017	751
Estimated aggregate amortization expense of intangible assets in 2018	748
Estimated aggregate amortization expense of intangible assets in 2019	727
Estimated aggregate amortization expense of intangible assets in 2020	690
Estimated aggregate amortization expense of intangible assets in 2021	$ 582