Share Tracking Awards Plans - General (Details) - USD ($) $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Share tracking award plans (STAP)
Awards granted under the STAP
Grant expiration period from the grant date	10 years
Share-based awards, vesting period	4 years
Aggregate STAP liability	$ 146.0		$ 268.9
Long term portion of STAP liability	$ 0.9		$ 74.1
Weighted-average assumptions used to measure the fair value of the outstanding STAP awards:
Expected volatility (as a percent)	32.80%	35.50%
Risk-free interest rate (as a percent)	1.40%	0.90%
Expected term of awards (in years)	1 year 10 months 24 days	2 years 9 months 18 days
Expected dividend yield (as a percent)	0.00%	0.00%
Closing price of common stock (in dollars per share)	$ 117.19	$ 118.08	$ 143.43
Share-based awards activity
Outstanding at the beginning of the year (in shares)	5,113,838
Exercised (in shares)	(741,859)
Forfeited (in shares)	(116,418)
Outstanding at the end of the year (in shares)	4,255,561
Exercisable at the end of the year (in shares)	2,553,425
Unvested at the end of the year (in shares)	1,702,136
Share awards, Weighted Average Exercise Price
Outstanding at the beginning of the year (in dollars per share)	$ 91.51
Exercised (in dollars per share)	70.70
Forfeited (in dollars per share)	110.40
Outstanding at the end of the year (in dollars per share)	94.62
Exercisable at the end of the year (in dollars per share)	96.99
Unvested at the end of the year (in dollars per share)	$ 91.06
Share awards, Weighted Average Remaining Contractual Term (in Years)
Outstanding at the end of the year	5 years 9 months 18 days
Exercisable at the end of the year	5 years 8 months 12 days
Unvested at the end of the year	6 years 1 month 6 days
Share awards, Aggregate Intrinsic Value
Outstanding at the end of the year (in dollars)	$ 153.1
Exercisable at the end of the year (in dollars)	86.6
Unvested at the end of the year (in dollars)	$ 66.5
Tranche one | Share tracking award plans (STAP)
Awards granted under the STAP
Share-based awards, vesting period	1 year
Decrease in non-current liabilities	$ 60.7
ASU 2016-09
Awards granted under the STAP
Cumulative effect on retained earnings, Net of tax	(5.8)
ASU 2016-09 | Share tracking award plans (STAP)
Awards granted under the STAP
Cumulative effect on aggregate STAP liability	8.4
Cumulative effect on retained earnings, Net of tax	$ (5.4)