Short-term Debt and Other Financing Agreements Short-term Debt and Other Financing Agreements (Details) (USD $) In Millions, unless otherwise specified									1 Months Ended		12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013 Line of Credit [Member]	Dec. 31, 2013 Letter of Credit [Member]	Dec. 31, 2012 Letter of Credit [Member]	Dec. 31, 2013 Tax exempt bond obligations and commodity contract collateral requirement [Member]	Dec. 31, 2012 Tax exempt bond obligations and commodity contract collateral requirement [Member]	Dec. 31, 2013 Certain transactions required by third parties [Member]	Mar. 31, 2013 Unsecured credit facility, $630 million, expiring July 2013 [Member] Line of Credit [Member]	Mar. 31, 2013 Unsecured credit facility, $600 million, expiring March 2018 [Member] Line of Credit [Member]	Dec. 31, 2013 Unsecured credit facility, $600 million, expiring June 2017 [Member] Line of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 1,200	$ 1,230								$ 600	$ 600
Line of Credit Facility, Amount Outstanding	0	0
Line of Credit Facility, Amounts Supported	(321)	(602)		(270)	(602)
Line of Credit Facility, Remaining Borrowing Capacity	879	628
Line of credit facility, borrowing capacity replaced									630
Line of Credit Facility, Expiration Date									Jul. 31, 2013	Mar. 31, 2018	Jun. 30, 2017
Debt to capitalization ratio			0.65
Letters of Credit Outstanding, Amount						$ 559	$ 602	$ 16