Accumulated Other Comprehensive Income (Loss) (Income (Expense) Effect of Reclassifications) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Affected Line Item on the Statements of Income [Line Items]
Wholesale energy	$ 389	$ 561	$ 1,189	$ 1,237
Fuel and energy purchases	(347)	(382)	(838)	(897)
Other Income (Expense) - net	6	3	12	10
Total Pre-tax	(6)	(17)	251	132
Income Taxes	3	43	(103)	(10)
Reclassification out of AOCI
Affected Line Item on the Statements of Income [Line Items]
Income (Loss) from Continuing Operations After Income Taxes	3	3	2	4
Reclassification out of AOCI | Available-for-sale securities
Affected Line Item on the Statements of Income [Line Items]
Other Income (Expense) - net	6	2	4	4
Income Taxes	(3)	(1)	(2)	(2)
Income (Loss) from Continuing Operations After Income Taxes	3	1	2	2
Reclassification out of AOCI | Qualifying derivatives
Affected Line Item on the Statements of Income [Line Items]
Total Pre-tax	7	10	12	17
Income Taxes	(3)	(4)	(5)	(7)
Income (Loss) from Continuing Operations After Income Taxes	4	6	7	10
Reclassification out of AOCI | Qualifying derivatives | Commodity contracts
Affected Line Item on the Statements of Income [Line Items]
Wholesale energy	(1)	2	(4)	0
Fuel and energy purchases	7	8	15	16
Other Income (Expense) - net	1	0	1	1
Reclassification out of AOCI | Defined benefit plans
Affected Line Item on the Statements of Income [Line Items]
Total Pre-tax	(6)	(7)	(11)	(14)
Income Taxes	2	3	4	6
Income (Loss) from Continuing Operations After Income Taxes	(4)	(4)	(7)	(8)
Reclassification out of AOCI | Defined benefit plans | Prior service costs
Affected Line Item on the Statements of Income [Line Items]
Total Pre-tax	0	1	0	1
Reclassification out of AOCI | Defined benefit plans | Net actuarial loss
Affected Line Item on the Statements of Income [Line Items]
Total Pre-tax	$ (6)	$ (8)	$ (11)	$ (15)