Debt (Details) (USD $)				3 Months Ended	6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Mar. 31, 2013 Revolving credit facility [Member]	Jun. 30, 2013 Revolving credit facility [Member]	Jun. 30, 2013 Commercial paper [Member]	Dec. 31, 2012 Commercial paper [Member]	Jun. 30, 2012 Commercial paper [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity				$ 500,000,000	$ 750,000,000
Debt Instrument Maturity Dates				8/31/2016	4/19/2018
Consolidated Funded Debt Percentage Of Consolidated Capital Permitted Under Credit Arrangement Amendment					70.00%
Amounts borrowed under short-term financing arrangements	$ 206,000,000	$ 263,000,000	$ 190,000,000		$ 0	$ 206,000,000	$ 263,000,000	$ 190,000,000