Accumulated Other Comprehensive Income (Loss), Net of Tax (Changes of AOCI) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	$ (95)	$ (388)	$ (388)
Other Comprehensive Income before Reclassifications	6	27
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	2	6
Other Comprehensive Income (Loss), net of tax	8	33
Accumulated Other Comprehensive Income (Loss), Ending Balance	(87)	(355)
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(2)	7	7
Other Comprehensive Income before Reclassifications	(5)	0	(2)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	7	(4)	(7)
Other Comprehensive Income (Loss), net of tax	2	(4)
Accumulated Other Comprehensive Income (Loss), Ending Balance	0	3	(2)
Pension and OPEB Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(238)	(485)	(485)
Other Comprehensive Income before Reclassifications	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	4	10
Other Comprehensive Income (Loss), net of tax	4	10
Accumulated Other Comprehensive Income (Loss), Ending Balance	(234)	(475)
Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	145	90	90
Other Comprehensive Income before Reclassifications	11	27
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(9)	0
Other Comprehensive Income (Loss), net of tax	2	27
Accumulated Other Comprehensive Income (Loss), Ending Balance	147	117
Power [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(63)	(328)	(328)
Other Comprehensive Income before Reclassifications	4	27
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	2	5
Other Comprehensive Income (Loss), net of tax	6	32
Accumulated Other Comprehensive Income (Loss), Ending Balance	(57)	(296)
Power [Member] | Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(1)	9	9
Other Comprehensive Income before Reclassifications	(6)	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	7	(4)
Other Comprehensive Income (Loss), net of tax	1	(4)
Accumulated Other Comprehensive Income (Loss), Ending Balance	0	5
Power [Member] | Pension and OPEB Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(204)	(422)	(422)
Other Comprehensive Income before Reclassifications	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	3	9
Other Comprehensive Income (Loss), net of tax	3	9
Accumulated Other Comprehensive Income (Loss), Ending Balance	(201)	(413)
Power [Member] | Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	142	85	85
Other Comprehensive Income before Reclassifications	10	27
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(8)	0
Other Comprehensive Income (Loss), net of tax	2	27
Accumulated Other Comprehensive Income (Loss), Ending Balance	$ 144	$ 112