Share-Based Compensation - STAP awards (Details) - STAP awards - USD ($) $ / shares in Units, $ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Awards granted
Grant expiration period from the grant date	10 years
Aggregate STAP liability	$ 96.8		$ 96.8
Weighted-average assumptions used to measure the fair value of the outstanding STAP awards:
Expected term of awards (in years)	1 year 4 months 24 days	1 year 10 months 24 days
Expected volatility	31.20%	32.10%
Risk-free interest rate (as a percent)	0.20%	0.20%
Expected dividend yield	0.00%	0.00%
Closing price of common stock (in dollars per share)	$ 179.41	$ 121.00	$ 151.79
Number of Awards
Outstanding at beginning of period (in shares)	2,121,860
Granted (in shares)	0
Exercised (in shares)	(687,372)
Forfeited/cancelled (in shares)	(562)
Outstanding at end of period (in shares)	1,433,926
Exercisable at end of period (in shares)	1,423,926
Unvested at end of period (in shares)	10,000
Weighted Average Grant Date Fair Value
Outstanding at beginning of period (in dollars per share)	$ 118.48
Granted (in dollars per share)	0
Exercised (in dollars per share)	122.70
Forfeited (in dollars per share)	145.30
Outstanding at end of period (in dollars per share)	116.45
Exercisable at end of period (in dollars per share)	116.90
Unvested at end of period (in dollars per share)	$ 52.57
Weighted Average Remaining Contractual Term (in Years)
Outstanding at end of the period	2 years 10 months 24 days
Exercisable at end of period	2 years 10 months 24 days
Unvested at end of period	1 year 4 months 24 days
Aggregate Intrinsic Value (in millions)
Outstanding at end of period	$ 90.3
Exercisable at end of period	89.0
Unvested at end of period	$ 1.3