Accumulated Other Comprehensive Income (Loss) - Income (Expense) Effect of Reclassifications (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Affected Line Item on the Statements of Income [Line Items]
Wholesale energy									$ 2,828	$ 2,653	$ 2,890
Energy purchases									(676)	(1,054)	(1,153)
Other Income (Expense) - net									(118)	30	32
Total Pre-tax									(368)	303	(420)
Income Taxes									27	(116)	159
Income (Loss) from Continuing Operations After Income Taxes	$ (62)	$ (401)	$ 26	$ 96	$ 149	$ 94	$ 2	$ (58)	(341)	187	$ (261)
Reclassification out of AOCI
Affected Line Item on the Statements of Income [Line Items]
Income (Loss) from Continuing Operations After Income Taxes									40	23
Reclassification out of AOCI | Available-for-sale securities
Affected Line Item on the Statements of Income [Line Items]
Other Income (Expense) - net									4	13
Income Taxes									(2)	(7)
Income (Loss) from Continuing Operations After Income Taxes									2	6
Reclassification out of AOCI | Qualifying derivatives
Affected Line Item on the Statements of Income [Line Items]
Total Pre-tax									31	42
Income Taxes									(12)	(17)
Income (Loss) from Continuing Operations After Income Taxes									19	25
Reclassification out of AOCI | Qualifying derivatives | Commodity contracts
Affected Line Item on the Statements of Income [Line Items]
Wholesale energy									(3)	1
Energy purchases									33	31
Discontinued operations									0	8
Other Income (Expense) - net									1	2
Reclassification out of AOCI | Defined benefit plans
Affected Line Item on the Statements of Income [Line Items]
Total Pre-tax									30	(13)
Income Taxes									(11)	5
Income (Loss) from Continuing Operations After Income Taxes									19	(8)
Reclassification out of AOCI | Defined benefit plans | Prior service costs
Affected Line Item on the Statements of Income [Line Items]
Total Pre-tax									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									$ 29	$ (9)