(Commercial Paper Program) (Details) (USD $)	1 Months Ended	3 Months Ended
	Dec. 31, 2012	Mar. 31, 2013	Mar. 31, 2012
Line of Credit Facility [Line Items]
Size of total commercial paper program		$ 1,200,000,000
Commitment fee, minimum percentage to pay depending on credit rating		0.15%
Commitment fee, maximum percentage to pay depending on credit rating		0.35%
Commitment fee		0.25%
Commitment fee discription		15 to 35 basis points, depending on credit rating 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
Line of credit amount outstanding	457,000,000
Line of credit, interest rate	0.48%
Line of credit, term at period end	28 days
Long-term Debt, Fair Value	900,000,000	900,000,000
3 Year Facility [Member]
Line of Credit Facility [Line Items]
Size of total commercial paper program		$ 1,200,000,000
Maximum [Member]
Line of Credit Facility [Line Items]
Indebtedness to cash flow		4