Intangible Assets and Goodwill - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2016	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible assets, net	$ 19,929,000		$ 19,929,000			$ 21,923,000
Amortization expense	1,000,000	$ 1,100,000	2,000,000	$ 2,200,000
Goodwill impairment	35,633,000	$ 0	35,633,000	$ 0	$ 37,100,000
Goodwill	$ 0		$ 0			$ 35,633,000