Goodwill and Intangible Assets - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill and Intangible Assets Disclosure [Abstract]
Aggregate intangible asset amortization expense	$ 1,320	$ 1,305	$ 1,327
Estimated intangible asset amortization expense in 2018	1,300
Estimated intangible asset amortization expense in 2019	1,300
Estimated intangible asset amortization expense in 2020	1,200
Estimated intangible asset amortization expense in 2021	1,200
Estimated intangible asset amortization expense in 2022	$ 1,200