Debt (Details 2) (USD $)	1 Months Ended
	Apr. 30, 2013 Time_Per_Annum	Sep. 30, 2013	Dec. 31, 2012
increase in commercial paper maximum
Debt
Amount outstanding		$ 500,000,000.0
Commercial Paper
Debt
Effective interest rate (as a percent)		0.30%
Maximum
Debt
Amount outstanding		1,500,000,000.000
commercial paper carrying value
Debt
Amount outstanding		405,900,000	308,500,000
Senior Unsecured Credit Facility
Debt
Maximum borrowing capacity	1,000,000,000.000	1,500,000,000.000
Maximum number of credit facility increases	1
Maximum borrowing capacity, subject to certain conditions	1,750,000,000.000
Outstanding uncommitted facility		$ 0	$ 0