Share-Based Compensation - STAP awards (Details) - STAP awards - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Awards granted
Grant expiration period from the grant date	4 years
Share-based awards, vesting period	1 year
Aggregate STAP liability	$ 77.5		$ 72.2
Weighted-average assumptions used to measure the fair value of the outstanding STAP awards:
Expected volatility (as a percent)	29.30%	33.30%
Risk-free interest rate (as a percent)	2.20%	2.10%
Expected term of awards (in years)	2 years 4 months 24 days	1 year 2 months 12 days
Closing price of common stock (in dollars per share)	$ 117.37	$ 112.36	$ 108.90
Share-based awards activity
Outstanding at January 1, 2018	2,867,979
Exercised (in shares)	(109,667)
Forfeited (in shares)	(65,067)
Outstanding at December 31, 2018	2,693,245
Exercisable at December 31, 2018	2,675,745
Unvested at December 31, 2018	17,500
Share awards, Weighted Average Exercise Price
Outstanding at January 1, 2018	$ 107.85
Exercised (in dollars per share)	61.19
Forfeited (in dollars per share)	163.22
Outstanding at December 31, 2018	108.42
Exercisable at December 31, 2018	108.41
Unvested at December 31, 2018	$ 109.29
Share awards, Weighted Average Remaining Contractual Term (in Years)
Outstanding at December 31, 2018	4 years 9 months 18 days
Exercisable at December 31, 2018	4 years 9 months 18 days
Unvested at December 31, 2018	4 years 9 months 18 days
Share awards, Aggregate Intrinsic Value
Outstanding at December 31, 2018	$ 72.9
Exercisable at December 31, 2018	72.3
Unvested at December 31, 2018	$ 0.6