Intangible Assets and Goodwill - Additional Information (Detail) - USD ($)	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible assets, net	$ 18,988,000			$ 18,988,000			$ 21,923,000
Amortization expense	900,000		$ 1,000,000	2,900,000	$ 3,300,000
Goodwill impairment	$ 0	$ 35,600,000	$ 0	$ 35,633,000	$ 0	$ 37,100,000