Accumulated Other Comprehensive Income (Loss), Net of Tax (Changes of AOCI) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Jun. 30, 2016	Sep. 30, 2016	Sep. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	$ (252)	$ (277)	$ (295)	$ (295)	$ (283)
Other Comprehensive Income before Reclassifications	27	(46)		46	(43)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	7	24		31	27
Other Comprehensive Income (Loss), net of tax	34	(22)		77	(16)
Accumulated Other Comprehensive Income (Loss), Ending Balance	(218)	(299)	(252)	(218)	(299)
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	1	1	0	0	10
Other Comprehensive Income before Reclassifications	1	0	1	2	1
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0	0	0	0	(10)
Other Comprehensive Income (Loss), net of tax	1	0		2	(9)
Accumulated Other Comprehensive Income (Loss), Ending Balance	2	1	1	2	1
Pension and OPEB Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(370)	(395)	(386)	(386)	(411)
Other Comprehensive Income before Reclassifications	0	0		0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	9	9		25	25
Other Comprehensive Income (Loss), net of tax	9	9		25	25
Accumulated Other Comprehensive Income (Loss), Ending Balance	(361)	(386)	(370)	(361)	(386)
Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	117	117	91	91	118
Other Comprehensive Income before Reclassifications	26	(46)		44	(44)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(2)	15		6	12
Other Comprehensive Income (Loss), net of tax	24	(31)		50	(32)
Accumulated Other Comprehensive Income (Loss), Ending Balance	141	86	117	141	86
Power [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(201)	(223)	(240)	(240)	(228)
Other Comprehensive Income before Reclassifications	24	(43)		40	(40)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	5	21		28	23
Other Comprehensive Income (Loss), net of tax	29	(22)		68	(17)
Accumulated Other Comprehensive Income (Loss), Ending Balance	(172)	(245)	(201)	(172)	(245)
Power [Member] | Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	0	2	0	0	11
Other Comprehensive Income before Reclassifications	0	0		0	1
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0	0		0	(10)
Other Comprehensive Income (Loss), net of tax	0	0		0	(9)
Accumulated Other Comprehensive Income (Loss), Ending Balance	0	2	0	0	2
Power [Member] | Pension and OPEB Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(313)	(337)	(327)	(327)	(351)
Other Comprehensive Income before Reclassifications	0	0		0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	7	7		21	21
Other Comprehensive Income (Loss), net of tax	7	7		21	21
Accumulated Other Comprehensive Income (Loss), Ending Balance	(306)	(330)	(313)	(306)	(330)
Power [Member] | Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	112	112	87	87	112
Other Comprehensive Income before Reclassifications	24	(43)		40	(41)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(2)	14		7	12
Other Comprehensive Income (Loss), net of tax	22	(29)		47	(29)
Accumulated Other Comprehensive Income (Loss), Ending Balance	$ 134	$ 83	$ 112	$ 134	$ 83