Accumulated Other Comprehensive Income (Loss) (Income (Expense) Effect of Reclassifications) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Affected Line Item on the Statements of Income [Line Items]
Wholesale energy	$ 986	$ 1,109	$ 2,181	$ 203
Energy purchases	(298)	(708)	(588)	893
Other Income (Expense) - net	1	10	11	23
Total Pre-tax	(300)	168	(169)	54
Income Taxes	(39)	(74)	(49)	(16)
Income (Loss) from Continuing Operations After Income Taxes	(339)	94	(218)	38
Reclassification out of AOCI
Affected Line Item on the Statements of Income [Line Items]
Income (Loss) from Continuing Operations After Income Taxes	0	6	4	17
Reclassification out of AOCI | Available-for-sale securities
Affected Line Item on the Statements of Income [Line Items]
Other Income (Expense) - net	(1)	7	3	11
Income Taxes	0	(4)	(2)	(6)
Income (Loss) from Continuing Operations After Income Taxes	(1)	3	1	5
Reclassification out of AOCI | Qualifying derivatives
Affected Line Item on the Statements of Income [Line Items]
Total Pre-tax	6	7	23	29
Income Taxes	(2)	(2)	(9)	(11)
Income (Loss) from Continuing Operations After Income Taxes	4	5	14	18
Reclassification out of AOCI | Qualifying derivatives | Commodity contracts
Affected Line Item on the Statements of Income [Line Items]
Wholesale energy	(3)	(2)	(3)	(1)
Energy purchases	9	8	25	23
Discontinued operations	0	1	0	6
Other Income (Expense) - net	0	0	1	1
Reclassification out of AOCI | Defined benefit plans
Affected Line Item on the Statements of Income [Line Items]
Total Pre-tax	(5)	(3)	(19)	(10)
Income Taxes	2	1	8	4
Income (Loss) from Continuing Operations After Income Taxes	(3)	(2)	(11)	(6)
Reclassification out of AOCI | Defined benefit plans | Prior service costs
Affected Line Item on the Statements of Income [Line Items]
Total Pre-tax	0	(2)	1	(4)
Reclassification out of AOCI | Defined benefit plans | Net actuarial loss
Affected Line Item on the Statements of Income [Line Items]
Total Pre-tax	$ (5)	$ (1)	$ (20)	$ (6)