Commitments and Contingencies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments And Contingencies Disclosure [Abstract]
Total rent expense	$ 5,500,000	$ 4,500,000	$ 4,000,000
Long-term sales tax liability	2,600,000	3,100,000
Current sales tax liability for non-contingent amounts	9,000,000	$ 6,000,000
Contingent liabilities related to employment agreement	$ 0