Accumulated Other Comprehensive Income (Loss), Net of Tax (Changes of AOCI) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	$ (242)	$ (269)	$ (263)	$ (295)	$ (295)
Other Comprehensive Income before Reclassifications	23	7	53	19
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(7)	10	(16)	24
Other Comprehensive Income (Loss), net of tax	16	17	37	43
Accumulated Other Comprehensive Income (Loss), Ending Balance	(226)	(252)	(226)	(252)	(263)
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	2	2	2	0	0
Other Comprehensive Income before Reclassifications	0	(1)	0	1	2
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0	0	0	0	0
Other Comprehensive Income (Loss), net of tax	0	(1)	0	1
Accumulated Other Comprehensive Income (Loss), Ending Balance	2	1	2	1	2
Pension and OPEB Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(392)	(378)	(398)	(386)	(386)
Other Comprehensive Income before Reclassifications	0	0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	6	8	12	16
Other Comprehensive Income (Loss), net of tax	6	8	12	16
Accumulated Other Comprehensive Income (Loss), Ending Balance	(386)	(370)	(386)	(370)	(398)
Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	148	107	133	91	91
Other Comprehensive Income before Reclassifications	23	8	53	18
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(13)	2	(28)	8
Other Comprehensive Income (Loss), net of tax	10	10	25	26
Accumulated Other Comprehensive Income (Loss), Ending Balance	158	117	158	117	133
Power [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(187)	(217)	(211)	(240)	(240)
Other Comprehensive Income before Reclassifications	22	6	50	16
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(7)	10	(11)	23
Other Comprehensive Income (Loss), net of tax	15	16	39	39
Accumulated Other Comprehensive Income (Loss), Ending Balance	(172)	(201)	(172)	(201)	(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	(335)	(320)	(340)	(327)	(327)
Other Comprehensive Income before Reclassifications	0	0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	5	7	10	14
Other Comprehensive Income (Loss), net of tax	5	7	10	14
Accumulated Other Comprehensive Income (Loss), Ending Balance	(330)	(313)	(330)	(313)	(340)
Power [Member] | Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	148	103	129	87	87
Other Comprehensive Income before Reclassifications	22	6	50	16
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(12)	3	(21)	9
Other Comprehensive Income (Loss), net of tax	10	9	29	25
Accumulated Other Comprehensive Income (Loss), Ending Balance	$ 158	$ 112	$ 158	$ 112	$ 129