Stock-based Compensation (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended	9 Months Ended				3 Months Ended	9 Months Ended
	Sep. 30, 2011 Restricted Stock Units [Member]	Sep. 30, 2011 Restricted Stock Units [Member]	Sep. 30, 2010 Restricted Stock Units [Member]	Sep. 30, 2011 Stock Options [Member]	Sep. 30, 2010 Stock Options [Member]	Mar. 31, 2011 Save As You Earn (SAYE) Plan [Member]	Sep. 30, 2011 Save As You Earn (SAYE) Plan [Member]	Sep. 30, 2011 Employee Stock Purchase Plan [Member]
Restricted stock unit activity [Roll Forward]
Unvested at beginning of period (in shares)	2,076,300	2,085,600
Granted (in shares)	0	397,800
Vested (in shares)	(730,300)	(1,100,800)
Forfeited (in shares)	(4,300)	(40,900)
Unvested at end of period (in shares)	1,341,700	1,341,700
Unvested shares expected to vest (in shares)	1,303,000	1,303,000
Restricted stock unit activity, additional disclosures [Abstract]
Weighted average grant date fair value, beginning of period (in dollars per share)	$ 58.04	$ 50.47
Weighted average grant date fair value, granted (in dollars per share)	$ 0	$ 89.46
Weighted average grant date fair value, vested (in dollars per share)	$ 42.94	$ 45.04
Weighted average grant date fair value, forfeited (in dollars per share)	$ 51.16	$ 57.40
Weighted average grant date fair value, end of period (in dollars per share)	$ 66.28	$ 66.28
Weighted average grant date fair value, unvested shares expected to vest (in dollars per share)	$ 66.15	$ 66.15
Weighted average remaining contractual life, unvested (in years)	2.07	2.07
Weighted average remaining contractual life, unvested shares expected to vest (in years)	2.07	2.07
Aggregate intrinsic value, unvested	$ 128.7	$ 128.7
Aggregate intrinsic value, unvested shares expected to vest	125.0	125.0
Unamortized deferred compensation cost	33.7	33.7
Fair value of shares vested		49.6	53.5
Stock Option Awards [Abstract]
Vesting period, maximum (in years)				5Y
Options outstanding (in shares)				10,000
Weighted average price of options outstanding (in dollars per share)				$ 18.73
Options exercised (in shares)				17,000	15,000
Intrinsic value of exercised options				1.3	1.0
Cash received from option exercises				$ 0.3	$ 0.3
Other Stock Compensation Programs [Abstract]
Discount on purchase price (in hundredths)							15.00%
Options to purchase discounted stock, vesting period (in years)							three or five
Options issued (in shares)						17,000
Options issued, exercise price (in dollars per share)						$ 83.72
Options outstanding							316,000
Employee Stock Purchase Program [Abstract]
Length of program period (in months)								1M