Accumulated Other Comprehensive Income (Loss), Net of Tax (Changes of AOCI) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	$ (295)	$ (283)	$ (283)
Other Comprehensive Income before Reclassifications	12	17
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	14	(4)
Other Comprehensive Income (Loss), net of tax	26	13
Accumulated Other Comprehensive Income (Loss), Ending Balance	(269)	(270)	(295)
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	0	10	10
Other Comprehensive Income before Reclassifications	2	1	2
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0	(10)	(12)
Other Comprehensive Income (Loss), net of tax	2	(9)
Accumulated Other Comprehensive Income (Loss), Ending Balance	2	1	0
Pension and OPEB Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(386)	(411)	(411)
Other Comprehensive Income before Reclassifications	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	8	8
Other Comprehensive Income (Loss), net of tax	8	8
Accumulated Other Comprehensive Income (Loss), Ending Balance	(378)	(403)	(386)
Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	91	118	118
Other Comprehensive Income before Reclassifications	10	16
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	6	(2)
Other Comprehensive Income (Loss), net of tax	16	14
Accumulated Other Comprehensive Income (Loss), Ending Balance	107	132	91
Power [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(240)	(228)	(228)
Other Comprehensive Income before Reclassifications	10	17
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	13	(5)
Other Comprehensive Income (Loss), net of tax	23	12
Accumulated Other Comprehensive Income (Loss), Ending Balance	(217)	(216)	(240)
Power [Member] | Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	0	11	11
Other Comprehensive Income before Reclassifications	0	1
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0	(10)
Other Comprehensive Income (Loss), net of tax	0	(9)
Accumulated Other Comprehensive Income (Loss), Ending Balance	0	2	0
Power [Member] | Pension and OPEB Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(327)	(351)	(351)
Other Comprehensive Income before Reclassifications	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	7	7
Other Comprehensive Income (Loss), net of tax	7	7
Accumulated Other Comprehensive Income (Loss), Ending Balance	(320)	(344)	(327)
Power [Member] | Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	87	112	112
Other Comprehensive Income before Reclassifications	10	16
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	6	(2)
Other Comprehensive Income (Loss), net of tax	16	14
Accumulated Other Comprehensive Income (Loss), Ending Balance	$ 103	$ 126	$ 87