Share Tracking Award Plans (Details) (USD $)	9 Months Ended
	Sep. 30, 2012 Share tracking award plans (STAP)	Sep. 30, 2011 Share tracking award plans (STAP)	Dec. 31, 2011 Share tracking award plans (STAP)	Oct. 31, 2012 2011 share tracking awards plan (STAP)	Feb. 29, 2012 2011 share tracking awards plan (STAP)	Feb. 29, 2012 2008 share tracking awards plan (STAP)
Awards granted under the STAP
Increase in total number of awards available for grant (in shares)				1,800,000	2,000,000
Decrease in total number of awards available for grant (in shares)						400,000
Share-based awards, vesting period	4 years
Grant expiration period from the grant date	10 years
Equity based compensation liability	$ 88,700,000		$ 79,900,000
Assumptions used to measure the fair value of STAP awards
Expected volatility (as a percent)	35.80%	46.60%
Risk-free interest rate (as a percent)	0.50%	0.60%
Expected term of awards	3 years 10 months 24 days	4 years 1 month 6 days
Forfeiture rate (as a percent)	7.00%	6.80%
Expected dividend yield (as a percent)	0.00%	0.00%
Share-based awards activity
Outstanding at the beginning of the period (in shares)	7,795,000
Granted (in shares)	1,833,353
Exercised (in shares)	(1,190,869)
Forfeited (in shares)	(246,961)
Outstanding at the end of the period (in shares)	8,190,523
Exercisable at the end of the period (in shares)	4,278,314
Expected to vest at the end of the period (in shares)	3,311,741
Share awards, Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 45.90
Granted (in dollars per share)	$ 48.01
Exercised (in dollars per share)	$ 29.02
Forfeited (in dollars per share)	$ 53.89
Outstanding at the end of the period (in dollars per share)	$ 48.59
Exercisable at the end of the period (in dollars per share)	$ 42.96
Expected to vest at the end of the period (in dollars per share)	$ 53.59
Share awards, Weighted Average Remaining Contractual Term
Outstanding at the end the period	7 years 6 months
Exercisable at the end of the period	6 years 10 months 24 days
Expected to vest at the end of the period	8 years 9 months 18 days
Share awards, Aggregate Intrinsic Value
Outstanding at the end of the period (in dollars)	75,018,000
Exercisable at the end of the period (in dollars)	58,512,000
Expected to vest at the end of the period (in dollars)	$ 15,391,000
STAP awards granted during the period, weighted average fair value (in dollars per share)	$ 21.26	$ 28.03