Accumulated Other Comprehensive Income (Loss), Net of Tax (Changes of AOCI) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	$ (226)	$ (252)	$ (263)	$ (295)	$ (295)
Other Comprehensive Income before Reclassifications	25	27	78	46
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(3)	7	(19)	31
Other Comprehensive Income (Loss), net of tax	22	34	59	77
Accumulated Other Comprehensive Income (Loss), Ending Balance	(204)	(218)	(204)	(218)	(263)
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	2	1	2	0	0
Other Comprehensive Income before Reclassifications	0	1	0	2	2
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(1)	0	(1)	0	0
Other Comprehensive Income (Loss), net of tax	(1)	1	(1)	2
Accumulated Other Comprehensive Income (Loss), Ending Balance	1	2	1	2	2
Pension and OPEB Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(386)	(370)	(398)	(386)	(386)
Other Comprehensive Income before Reclassifications	0	0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	6	9	18	25
Other Comprehensive Income (Loss), net of tax	6	9	18	25
Accumulated Other Comprehensive Income (Loss), Ending Balance	(380)	(361)	(380)	(361)	(398)
Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	158	117	133	91	91
Other Comprehensive Income before Reclassifications	25	26	78	44
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(8)	(2)	(36)	6
Other Comprehensive Income (Loss), net of tax	17	24	42	50
Accumulated Other Comprehensive Income (Loss), Ending Balance	175	141	175	141	133
Power [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(172)	(201)	(211)	(240)	(240)
Other Comprehensive Income before Reclassifications	24	24	74	40
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(4)	5	(15)	28
Other Comprehensive Income (Loss), net of tax	20	29	59	68
Accumulated Other Comprehensive Income (Loss), Ending Balance	(152)	(172)	(152)	(172)	(211)
Power [Member] | Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	0	0	0	0	0
Other Comprehensive Income before Reclassifications	0	0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0	0	0	0
Other Comprehensive Income (Loss), net of tax	0	0	0	0
Accumulated Other Comprehensive Income (Loss), Ending Balance	0	0	0	0	0
Power [Member] | Pension and OPEB Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(330)	(313)	(340)	(327)	(327)
Other Comprehensive Income before Reclassifications	0	0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	5	7	15	21
Other Comprehensive Income (Loss), net of tax	5	7	15	21
Accumulated Other Comprehensive Income (Loss), Ending Balance	(325)	(306)	(325)	(306)	(340)
Power [Member] | Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	158	112	129	87	87
Other Comprehensive Income before Reclassifications	24	24	74	40
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(9)	(2)	(30)	7
Other Comprehensive Income (Loss), net of tax	15	22	44	47
Accumulated Other Comprehensive Income (Loss), Ending Balance	$ 173	$ 134	$ 173	$ 134	$ 129