Equity Incentive Plans - Summary of Operational Milestone Based on Revenue or Adjusted EBITDA (Detail) - Chief Executive Officer [Member] - 2018 CEO Performance Award [Member] $ in Billions	9 Months Ended
	Sep. 30, 2018 USD ($)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total revenue of operational milestone, one	$ 20.0
Total revenue of operational milestone, two	35.0
Total revenue of operational milestone, three	55.0
Total revenue of operational milestone, four	75.0
Total revenue of operational milestone, five	100.0
Total revenue of operational milestone, six	125.0
Total revenue of operational milestone, seven	150.0
Total revenue of operational milestone, eight	175.0
Adjusted EBITDA of operational milestone, one	1.5
Adjusted EBITDA of operational milestone, two	3.0
Adjusted EBITDA of operational milestone, three	4.5
Adjusted EBITDA of operational milestone, four	6.0
Adjusted EBITDA of operational milestone, five	8.0
Adjusted EBITDA of operational milestone, six	10.0
Adjusted EBITDA of operational milestone, seven	12.0
Adjusted EBITDA of operational milestone, eight	$ 14.0