Stockholders' Equity - Employee Stock Options (Details) - Stock Options [Member] - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 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	39.00%	34.40%	34.40%	34.20%
Risk-free interest rate	0.30%	2.20%	1.10%	2.50%
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)			0.0
Exercised (in shares)			(0.4)
Balance - Ending (in shares)	2.9		2.9
Weighted- Average Exercise Price
Balance - Beginning (in dollars per share)			$ 50.37
Granted (in dollars per share)			118.50
Forfeited (in dollars per share)			80.97
Exercised (in dollars per share)			40.78
Balance - Ending (in dollars per share)	$ 66.01		$ 66.01
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Options vested and expected to vest, Outstanding (in shares)	2.9		2.9
Options vested and expected to vest, Weighted average exercise price (in dollars per share)	$ 66.01		$ 66.01
Options vested and expected to vest, Weighted average remaining contractual life (in years)			4 years 8 months 12 days
Options vested and expected to vest, Aggregate intrinsic value	$ 206.0		$ 206.0
Options exercisable, Outstanding (in shares)	1.4		1.4
Options exercisable, Weighted average exercise price (in dollars per share)	$ 41.09		$ 41.09
Options exercisable, Weighted average remaining contractual life (in years)			3 years 6 months
Options exercisable, Aggregate intrinsic value	$ 134.3		$ 134.3
Compensation cost not yet recognized	$ 38.5		$ 38.5
Compensation cost not yet recognized period of recognition			3 years
Weighted-average fair value per share granted	$ 46.92	$ 26.38	$ 35.51	$ 27.08
Intrinsic value of options exercised	$ 17.3	$ 9.6	$ 30.4	$ 36.8
Fair value of options vested	$ 2.4	$ 2.0	$ 8.4	$ 6.2

Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.