Stockholders' Equity - Stock Option Activity (Details) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 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
Granted (in shares) | shares	0
Exercised (in shares) | shares	(565)
Canceled (in shares) | shares	(134)
Balance, end of period (in shares) | shares	20,687
Options exercisable (in shares) | shares	20,638
Options vested and expected to vest (in shares) | shares	17,145
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)	$ 3.01
Granted, weighted average exercise price (in dollars per share)	0.00
Exercised, weighted average exercise price (in dollars per share)	0.02
Canceled, weighted average exercise price (in dollars per share)	0.05
Balance, weighted average exercise price, end of period (in dollars per share)	3.03
Options exercisable, weighted average exercise price (in dollars per share)	3.03
Options vested and expected to vest, weighted average exercise price (in dollars per share)	$ 2.62
Options outstanding, aggregate intrinsic value, end of period | $	$ 50,820
Options exercisable, aggregate intrinsic value | $	50,763
Options vested and expected to vest, aggregate intrinsic value | $	$ 47,238
Fair value of the Class A common stock (in dollars per share)	$ 5.10