(Commercial Paper Program) (Details) (USD $)	1 Months Ended	3 Months Ended		6 Months Ended					6 Months Ended
	Dec. 31, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 4 Year Facility [Member]	Jun. 19, 2013 4 Year Facility [Member]	Jun. 18, 2013 3 Year Facility [Member]	Jun. 30, 2013 Maximum [Member]
Line of Credit Facility [Line Items]
Size of total commercial paper program		$ 1,000,000,000		$ 1,000,000,000			$ 1,000,000,000	$ 1,200,000,000
Commitment fee, minimum percentage to pay depending on credit rating						0.20%
Commitment fee, maximum percentage to pay depending on credit rating						0.45%
Commitment fee						0.25%
Commitment fee discription						20 to 45 basis points for our credit facility, depending on our indebtedness to cash flow ratio, whether or not amounts have been borrowed and currently pay a commitment fee of 25 basis points.
Dividend declared per common share (in usd per share)	$ 2.50	$ 0.28	$ 0.255	$ 0.56	$ 0.51
Line of credit amount outstanding	457,000,000
Line of credit, interest rate	0.48%
Line of credit, term at period end	28 days
Indebtedness to cash flow									3.25
Long-term Debt, Fair Value	$ 900,000,000	$ 800,000,000		$ 800,000,000