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, 2015	Mar. 31, 2014	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	$ (283)	$ (95)	$ (95)
Other Comprehensive Income before Reclassifications	17	6
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(4)	2
Other Comprehensive Income (Loss), net of tax	13	8
Accumulated Other Comprehensive Income (Loss), Ending Balance	(270)	(87)
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	10	(2)	(2)
Other Comprehensive Income before Reclassifications	1	(5)	7
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(10)	7	5
Other Comprehensive Income (Loss), net of tax	(9)	2
Accumulated Other Comprehensive Income (Loss), Ending Balance	1	0	10
Pension and OPEB Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(411)	(238)	(238)
Other Comprehensive Income before Reclassifications	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	8	4
Other Comprehensive Income (Loss), net of tax	8	4
Accumulated Other Comprehensive Income (Loss), Ending Balance	(403)	(234)
Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	118	145	145
Other Comprehensive Income before Reclassifications	16	11
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(2)	(9)
Other Comprehensive Income (Loss), net of tax	14	2
Accumulated Other Comprehensive Income (Loss), Ending Balance	132	147
Power [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(228)	(63)	(63)
Other Comprehensive Income before Reclassifications	17	4
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(5)	2
Other Comprehensive Income (Loss), net of tax	12	6
Accumulated Other Comprehensive Income (Loss), Ending Balance	(216)	(57)
Power [Member] | Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	11	(1)	(1)
Other Comprehensive Income before Reclassifications	1	(6)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(10)	7
Other Comprehensive Income (Loss), net of tax	(9)	1
Accumulated Other Comprehensive Income (Loss), Ending Balance	2	0
Power [Member] | Pension and OPEB Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(351)	(204)	(204)
Other Comprehensive Income before Reclassifications	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	7	3
Other Comprehensive Income (Loss), net of tax	7	3
Accumulated Other Comprehensive Income (Loss), Ending Balance	(344)	(201)
Power [Member] | Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	112	142	142
Other Comprehensive Income before Reclassifications	16	10
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(2)	(8)
Other Comprehensive Income (Loss), net of tax	14	2
Accumulated Other Comprehensive Income (Loss), Ending Balance	$ 126	$ 144