Stock-Based Compensation and Employee Benefit Plan - Stock Option and Restricted Stock Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Options/ RSUs Available for Grant
Options/RSUs available for grant, balance (in shares)	1,037,616	2,768,450
Added to plan (in shares)	40,782,196	647,159
Granted (in shares)	(16,853,050)	(2,698,594)
Cancelled (in shares)	916,572	967,760
Expired (in shares)	(7,348,257)	(647,159)
Options/RSUs available for grant, balance (in shares)	18,535,077	1,037,616	2,768,450
Outstanding Options/RSUs, Number of Shares
Outstanding (in shares)	14,744,981	13,171,196
Granted (in shares)	16,853,050	2,698,594
Exercised (in shares)	(395,611)	(157,049)
Cancelled (in shares)	(916,572)	(967,760)
Outstanding (in shares)	30,285,848	14,744,981	13,171,196
Outstanding Options Weighted Average Exercise Price
Outstanding (in dollars per share)	$ 26.42	$ 23.85
Granted (in dollars per share)	16.24	30.96
Exercised (in dollars per share)	4.03	2.76
Cancelled (in dollars per share)	9.96	7.44
Outstanding (in dollars per share)	$ 25.88	$ 26.42	$ 23.85
Outstanding, remaining contractual life	6 years 9 months	6 years 2 months 9 days	6 years 1 month 10 days
Outstanding, aggregate intrinsic value	$ 110,750	$ 52,703	$ 74,717
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Vested and expected to vest (in shares)	13,038,789
Exercisable (in shares)	7,858,582
Vested and expected to vest, weighted average exercise price (in dollars per share)	$ 26.07
Exercisable, weighted average exercise price (in dollars per share)	$ 27.51
Vested and expected to vest, remaining contractual life	6 years 8 months
Exercisable, remaining contractual life	5 years 1 month 25 days
Vested and expected to vest, aggregate intrinsic value	$ 104,482
Exercisable, aggregate intrinsic value	$ 51,608