Commitments and Contingencies (Details) (USD $) In Millions, except Share data	0 Months Ended		1 Months Ended			9 Months Ended
	Oct. 25, 2011	Oct. 24, 2011	Feb. 28, 2011	Jul. 31, 2010	Dec. 31, 2010	Sep. 30, 2011
Stock Repurchases
Amount authorized under stock repurchase plan			$ 450	$ 350
Increase in amount authorized under stock repurchase plan	200		100
Number of shares repurchased (in shares)		517,705			1,684,510	6,728,222
Average price of shares repurchased (in dollars per share)					$ 48.36	$ 48.74
Aggregate cost of shares repurchased		$ 25.6			$ 81.5	$ 327.9