Short-term Debt and Other Financing Agreements Short-term Debt and Other Financing Agreements (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 1,200	$ 1,200
Short-term debt	(20)	0
Line of Credit Facility, Amounts Supported	(398)	(321)
Line of Credit Facility, Remaining Borrowing Capacity	782	879
Line of Credit [Member]
Line of Credit Facility [Line Items]
Debt to capitalization ratio	0.65
Commercial Paper [Member]
Line of Credit Facility [Line Items]
Debt, Weighted Average Interest Rate	0.43%
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Amounts Supported	(270)	(270)
Tax exempt bond obligations and commodity contract collateral requirement [Member]
Line of Credit Facility [Line Items]
Letters of Credit Outstanding, Amount	451	559
Certain transactions required by third parties [Member]
Line of Credit Facility [Line Items]
Letters of Credit Outstanding, Amount	16
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	600
Unsecured credit facility, $600 million, expiring March 2018 [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 600