Stock-based Compensation - Option Activity (Details) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Number of Shares Subject to Outstanding Options
Outstanding, beginning of period (shares) | shares	3,890
Granted (shares) | shares	856
Exercised (shares) | shares	(86)
Forfeited (shares) | shares	(81)
Outstanding, end of period (shares) | shares	4,579
Options exercisable (shares) | shares	3,006
Options vested and expected to vest (shares) | shares	4,372
Weighted-average Exercise Price of Outstanding Options
Outstanding, beginning of period (usd per share) | $ / shares	$ 4.40
Granted (usd per share) | $ / shares	4.57
Exercised (usd per share) | $ / shares	3.10
Forfeited/Expired (usd per share) | $ / shares	7.72
Outstanding, end of period (usd per share) | $ / shares	4.40
Options exercisable (usd per share) | $ / shares	4.48
Options vested and expected to vest (usd per share) | $ / shares	$ 4.40
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted average remaining contractual terms	6 years 4 months 13 days
Weighted average remaining contractual terms, exercisable options	5 years 2 months 16 days
Weighted average remaining contractual terms, vested and expected to vest options	6 years 3 months 4 days
Aggregate intrinsic value | $	$ 19,188
Aggregate intrinsic value, exercisable options | $	12,722
Aggregate intrinsic value, options vested and expected to vest | $	$ 18,357