Stock-Based Compensation - Stock Options (Details) - Employee and non-employee stock options - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock Based Compensation
Shares available for grant, beginning balance	4,115
Additional shares authorized	2,107
Options granted (in shares)	(2,078)
Options forfeited (in shares)	593
Shares available for grant, end balance	4,737	4,115
Number of shares
Outstanding, beginning balance (in shares)	9,043
Options granted (in shares)	2,078
Options exercised (in shares)	(565)
Options forfeited (in shares)	(593)
Outstanding, end balance (in shares)	9,963	9,043
Exercisable (in shares)	6,103
Vested and expected to vest (in shares)	9,592
Weighted-Average Exercise Price
Outstanding, beginning balance (in dollars per share)	$ 5.72
Granted (in dollars per share)	10.55
Exercised (in dollars per share)	4.10
Forfeited (in dollars per share)	10.37
Outstanding, end balance (in dollars per share)	6.54	$ 5.72
Exercisable (in dollars per share)	4.41
Vested and expected to vest (in dollars per share)	$ 6.41
Additional disclosures
Weighted average contractual term, options outstanding	7 years 11 days	7 years 6 months 18 days
Exercisable (in years)	5 years 11 months 12 days
Vested and expected to vest (in years)	6 years 11 months 16 days
Aggregate intrinsic value, options outstanding	$ 31,902	$ 55,396
Aggregate intrinsic value, options exercisable	30,201
Aggregate intrinsic value, vested and expected to vest	31,738
Aggregate intrinsic value, options exercised	3,700	16,900	$ 6,900
Fair value, options vested	$ 12,100	$ 8,500	$ 4,500
Weighted-average grant date fair value, options granted	$ 5.71	$ 5.92	$ 7.29