Share-Based Compensation - Stock Option Activity (Details) - Options to purchase common stock - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	6 Months Ended
	Jun. 30, 2020
Number of Shares
Options outstanding (in shares)	16,724
Options, Granted (in shares)	4,225
Options, Exercised (in shares)	(1,663)
Options, Forfeited (in shares)	(497)
Options, Expired (in shares)	(375)
Options outstanding (in shares)	18,414
Options, Vested and unvested expected to vest (in shares)	17,144
Options, Exercisable (in shares)	10,660
Weighted Average Exercise  Price
Options outstanding (in dollars per share)	$ 9.15
Options, Granted (in dollars per share)	9.72
Options, Exercised (in dollars per share)	6.45
Options, Forfeited (in dollars per share)	10.33
Options, Expired (in dollars per share)	13.31
Options outstanding (in dollars per share)	9.40
Options, Vested and unvested expected to vest (in dollars per share)	9.35
Options, Exercisable (in dollars per share)	$ 8.76
Additional Disclosures
Weighted average remaining contractual life, options outstanding	6 years 10 months 24 days
Weighted average remaining contractual life, options vested and unvested expected to vest	6 years 8 months 12 days
Weighted average remaining contractual life, options exercisable	5 years 4 months 24 days
Aggregate intrinsic value, options outstanding	$ 105.6
Aggregate intrinsic value, vested and unvested expected to vest	99.4
Aggregate intrinsic value, exercisable	$ 68.2