Share Tracking Award Plans (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 30, 2014
Share tracking award plans (STAP)
Awards granted under the STAP
Share-based awards, vesting period	4 years
Grant expiration period from the grant date	10 years
Aggregate STAP liability	$ 322,700,000
Long term portion of STAP liability	40,600,000
Expected volatility maximum period	5 years
Assumptions used to measure the fair value of STAP awards
Expected volatility (as a percent)	34.00%	32.70%	32.80%
Risk-free interest rate (as a percent)	1.30%	1.10%	0.50%
Expected term of awards	4 years	3 years 10 months 24 days	3 years 8 months 12 days
Expected forfeiture rate (as a percent)	9.30%	10.10%	8.70%
Expected dividend yield (as a percent)	0.00%	0.00%	0.00%
Share-based awards activity
Outstanding at the beginning of the year (in shares)	8,734,901
Granted (in shares)	1,604,525
Exercised (in shares)	(2,315,093)
Forfeited (in shares)	(307,909)
Outstanding at the end of the year (in shares)	7,716,424	8,734,901
Exercisable at the end of the year (in shares)	2,618,117
Expected to vest at the end of the year (in shares)	4,615,370
Share awards, Weighted-Average Exercise Price
Outstanding at the beginning of the year (in dollars per share)	$ 52.75
Granted (in dollars per share)	$ 95.39
Exercised (in dollars per share)	$ 48.01
Forfeited (in dollars per share)	$ 63.96
Outstanding at the end of the year (in dollars per share)	$ 62.59	$ 52.75
Exercisable at the end of the year (in dollars per share)	$ 52.69
Expected to vest at the end of the year (in dollars per share)	$ 67.89
Share awards, Weighted Average Remaining Contractual Term
Outstanding at the end the year	7 years 4 months 24 days
Exercisable at the end of the year	5 years 10 months 24 days
Expected to vest at the end of the year	8 years 2 months 12 days
Share awards, Aggregate Intrinsic Value
Outstanding at the end of the year (in dollars)	516,222,000
Exercisable at the end of the year (in dollars)	201,075,000
Expected to vest at the end of the year (in dollars)	$ 284,296,000
Awards granted during the period, weighted average grant date fair value (in dollars per share)	$ 33.82	$ 24.78	$ 21.28
Share tracking award plans (STAP) | Minimum
Awards granted under the STAP
Share-based awards, vesting period	1 year
2011 share tracking awards plan (STAP)
Awards granted under the STAP
Number of awards remained available for grant	2,700,000
Additional increase in total number of awards available for grant (in shares)				3,000,000