Stockholders' Equity - Stock Option Activity (Details) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Balance, beginning of period (in shares) | shares	21,386
Balance, end of period (in shares) | shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]
Balance, weighted average exercise price, beginning of period (in dollars per share) | $ / shares	$ 3.01
Balance, weighted average exercise price, end of period (in dollars per share) | $ / shares
2007 And 2015 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Granted (in shares) | shares	0
Exercised (in shares) | shares	(4,375)
Canceled (in shares) | shares	(748)
Balance, end of period (in shares) | shares	16,263
Options exercisable (in shares) | shares	15,131
Options vested and expected to vest (in shares) | shares	16,253
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 0.00
Exercised, weighted average exercise price (in dollars per share) | $ / shares	2.29
Canceled, weighted average exercise price (in dollars per share) | $ / shares	7.33
Balance, weighted average exercise price, end of period (in dollars per share) | $ / shares	3.00
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	2.80
Options vested and expected to vest, weighted average exercise price (in dollars per share) | $ / shares	$ 3.00
Options outstanding, weighted average remaining contractual term	4 years 11 months 8 days
Options exercisable, weighted average remaining contractual term	4 years 9 months 18 days
Options vested and expected to vest, weighted average remaining contractual term	4 years 11 months 8 days
Options exercised, aggregate intrinsic value | $	$ 21,744
Options outstanding, aggregate intrinsic value | $	38,459
Options exercisable, aggregate intrinsic value | $	37,707
Options vested and expected to vest, aggregate intrinsic value | $	$ 38,455