Stock-based Compensation - Schedule of Stock Option Activity (Details) $ / shares in Units, shares in Millions, $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($) $ / shares shares
Awards (in millions)
Beginning Balance | shares	8.1
Grants in period | shares	1.1
Exercises in period | shares	(1.6)
Forfeitures in period | shares	(0.2)
Ending Balance | shares	7.4
Weighted Average Exercise Price (usd per share)
Beginning Balance | $ / shares	$ 16.54
Grants in period (usd per share) | $ / shares	29.75
Exercised (usd per share) | $ / shares	10.54
Forfeited (usd per share) | $ / shares	29.62
Ending Balance | $ / shares	$ 19.50
Vested and Expected to Vest
Vested and expected to vest, in shares | shares	7.4
Vested and expected to vest, weighted average exercise price (usd) | $ / shares	$ 19.50
Vested and expected to vest, aggregate intrinsic value | $	$ 75.1
Vested and expected to vest, weighted average contractual life (in years)	6 years 4 months 10 days
Exercisable
Exercisable, in shares | shares	5.5
Exercisable, usd per share | $ / shares	$ 16.39
Exercisable, aggregate intrinsic value | $	$ 73.4
Exercisable, weighted average contractual life (in years)	5 years 6 months 15 days