Accumulated Other Comprehensive Income (Loss) (Income (Expense) Effect of Reclassifications) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Affected Line Item on the Statements of Income [Line Items]
Wholesale energy	$ 673	$ 551	$ 1,195	$ (906)
Energy purchases	(289)	(203)	(290)	1,601
Other Income (Expense) - net	3	7	10	13
Total Pre-tax	(18)	(9)	131	(114)
Income Taxes	43	11	(10)	58
Income (Loss) from Continuing Operations After Income Taxes	25	2	121	(56)
Reclassification out of AOCI
Affected Line Item on the Statements of Income [Line Items]
Income (Loss) from Continuing Operations After Income Taxes	3	7	4	11
Reclassification out of AOCI | Available-for-sale securities
Affected Line Item on the Statements of Income [Line Items]
Other Income (Expense) - net	2	2	4	4
Total Pre-tax	2	2	4	4
Income Taxes	(1)	(1)	(2)	(2)
Income (Loss) from Continuing Operations After Income Taxes	1	1	2	2
Reclassification out of AOCI | Qualifying derivatives
Affected Line Item on the Statements of Income [Line Items]
Total Pre-tax	10	13	17	22
Income Taxes	(4)	(5)	(7)	(9)
Income (Loss) from Continuing Operations After Income Taxes	6	8	10	13
Reclassification out of AOCI | Qualifying derivatives | Commodity contracts
Affected Line Item on the Statements of Income [Line Items]
Wholesale energy	2	2	0	1
Energy purchases	8	8	16	15
Discontinued operations	0	3	0	5
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	(7)	(3)	(14)	(7)
Income Taxes	3	1	6	3
Income (Loss) from Continuing Operations After Income Taxes	(4)	(2)	(8)	(4)
Reclassification out of AOCI | Defined benefit plans | Prior service costs
Affected Line Item on the Statements of Income [Line Items]
Total Pre-tax	1	0	1	(2)
Reclassification out of AOCI | Defined benefit plans | Net actuarial loss
Affected Line Item on the Statements of Income [Line Items]
Total Pre-tax	$ (8)	$ (3)	$ (15)	$ (5)