Short-term Debt and Other Financing Agreements Short-term Debt and Other Financing Agreements (Details) (USD $) In Millions, unless otherwise specified								12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Line of Credit [Member]	Dec. 31, 2012 Letter of Credit [Member]	Dec. 31, 2011 Letter of Credit [Member]	Dec. 31, 2011 Commercial Paper [Member]	Jun. 30, 2012 Unsecured credit facility, $635 million, expiring October 2012 [Member] Line of Credit [Member]	Dec. 31, 2012 Unsecured credit facility, $600 million, expiring June 2017 [Member] Line of Credit [Member]	Dec. 31, 2012 Unsecured credit facility, $720 million, expiring July 2012 [Member] Line of Credit [Member]	Dec. 31, 2011 Unsecured credit facility, $720 million, expiring July 2012 [Member] Line of Credit [Member]	Dec. 31, 2012 Tax exempt bond obligations and commodity contract collateral requirement [Member]	Dec. 31, 2011 Tax exempt bond obligations and commodity contract collateral requirement [Member]	Dec. 31, 2012 Certain transactions required by third parties [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 1,230	$ 1,355						$ 600	$ 630	$ 720
Line of Credit Facility, Amount Outstanding	0	688
Letters of Credit Outstanding, Amount											602	601	14
Line of Credit Facility, Amounts Supported				(602)	(304)
Line of Credit Facility, Remaining Borrowing Capacity	628	363
Line of credit facility, borrowing capacity replaced							$ 635
Line of Credit Facility, Expiration Date								Jun. 30, 2017	Jul. 31, 2013	Jul. 31, 2012
Debt, Weighted Average Interest Rate						0.51%
Debt to capitalization ratio			0.65