Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Rent expense under long-term operating leases	$ 3,100	$ 3,200	$ 2,700
Rent expense to related parties	$ 144	$ 141	$ 143