Description of the Business and Basis of Presentation - Changes in Deferred Revenue (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Deferred Revenue Disclosure [Abstract]
Deferred revenue, including long-term portion, beginning balance	$ 90,437	$ 89,400
Additions	749,321	674,171
Revenue recognized during the period	(717,912)	(650,804)
Other adjustments	(3,187)	(8,296)
Deferred revenue, including long-term portion, ending balance	118,659	104,471
Long term deferred revenue	10,638	10,548
Deferred revenue, short-term portion	$ 108,021	$ 93,923