Stock-Based Compensation- Stock Option Activity (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Stock option activity
Outstanding, Beginning Balance, Number of Shares Underlying Options	1.1
Granted, Number of Shares Underlying Options	0
Exercised, Number of Shares Underlying Options	(0.1)
Outstanding, Ending Balance, Number of Shares Underlying Options	1.0	1.1
Exercisable, Number of Shares Underlying Options	0.9
Outstanding, Beginning Balance, Weighted Average Exercise Price per Share	$ 36.31
Granted, Weighted Average Exercise Price per Share	$ 0
Exercised, Weighted Average Exercise Price per Share	$ 20.30
Outstanding, Ending Balance, Weighted Average Exercise Price per Share	$ 37.52	$ 36.31
Exercisable, Weighted Average Exercise Price per Share	$ 36.61
Weighted average fair value of each option	$ 0
Outstanding, Beginning Balance, Weighted Average Remaining Contractual Life (In years)	3 years 6 months 0 days	4 years 0 months 0 days
Outstanding, Ending Balance, Weighted Average Remaining Contractual Life (In years)	3 years 6 months 0 days	4 years 0 months 0 days
Exercisable, Weighted Average Remaining Contractual Life (In years)	3 years 4 months 0 days
Outstanding, Beginning Balance, Aggregate Intrinsic Value	$ 7
Exercised, Aggregate Intrinsic Value	1
Outstanding, Ending Balance, Aggregate Intrinsic Value	2	7
Exercisable, Aggregate Intrinsic Value	$ 2