Stockholders' Equity - Employee Stock Options (Details) - Stock Options [Member] - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Expected term in years	4 years 4 months 24 days	4 years 4 months 24 days	4 years 4 months 24 days	4 years 4 months 24 days
Volatility	39.90%	35.30%	34.60%	34.20%
Risk-free interest rate	0.20%	1.40%	1.10%	2.40%
Dividend rate	0.00%	0.00%	0.00%	0.00%
Number of Shares
Balance - Beginning (in shares)			2.7
Granted (in shares)			0.6
Forfeited (in shares)			(0.1)
Exercised (in shares)			(0.4)
Balance - Ending (in shares)	2.8		2.8
Weighted- Average Exercise Price
Balance - Beginning (in dollars per share)			$ 50.37
Granted (in dollars per share)			118.96
Forfeited (in dollars per share)			91.73
Exercised (in dollars per share)			40.99
Balance - Ending (in dollars per share)	$ 66.72		$ 66.72
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Options vested and expected to vest, Outstanding (in shares)	2.8		2.8
Options vested and expected to vest, Weighted average exercise price (in dollars per share)	$ 66.72		$ 66.72
Options vested and expected to vest, Weighted average remaining contractual life (in years)			4 years 4 months 24 days
Options vested and expected to vest, Aggregate intrinsic value	$ 146.4		$ 146.4
Options exercisable, Outstanding (in shares)	1.5		1.5
Options exercisable, Weighted average exercise price (in dollars per share)	$ 42.81		$ 42.81
Options exercisable, Weighted average remaining contractual life (in years)			3 years 3 months 18 days
Options exercisable, Aggregate intrinsic value	$ 109.6		$ 109.6
Compensation cost not yet recognized	$ 35.1		$ 35.1
Compensation cost not yet recognized period of recognition			2 years 9 months 18 days
Weighted-average fair value per share granted	$ 43.80	$ 24.97	$ 35.76	$ 26.96
Intrinsic value of options exercised	$ 5.5	$ 3.2	$ 35.8	$ 40.1
Fair value of options vested	$ 2.5	$ 2.0	$ 10.9	$ 8.2