Stock-Based Compensation - Stock Options (Details) - Employee and non-employee stock options - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Stock Based Compensation
Shares available for grant, beginning balance	4,737
Additional shares authorized	2,162
Options granted (in shares)	(1,697)
Options forfeited (in shares)	759
Shares available for grant, end balance	5,961	4,737
Number of shares
Outstanding, beginning balance (in shares)	9,963
Options granted (in shares)	1,697
Options exercised (in shares)	(1,774)
Options forfeited (in shares)	(759)
Outstanding, end balance (in shares)	9,127	9,963
Exercisable (in shares)	5,348
Vested and expected to vest (in shares)	8,908
Weighted-Average Exercise Price
Outstanding, beginning balance (in dollars per share)	$ 6.54
Granted (in dollars per share)	11.24
Exercised (in dollars per share)	6.80
Forfeited (in dollars per share)	7.23
Outstanding, end balance (in dollars per share)	7.31	$ 6.54
Exercisable (in dollars per share)	5.01
Vested and expected to vest (in dollars per share)	$ 7.23
Additional disclosures
Weighted average contractual term, options outstanding	6 years 11 months 19 days	7 years 11 days
Exercisable (in years)	5 years 9 months
Vested and expected to vest (in years)	6 years 11 months 5 days
Aggregate intrinsic value, options outstanding	$ 151,881	$ 31,902
Aggregate intrinsic value, options exercisable	101,241
Aggregate intrinsic value, vested and expected to vest	$ 148,943