Accumulated Other Comprehensive Income (Loss), Net of Tax (Changes of AOCI) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	$ (263)	$ (295)	$ (295)
Other Comprehensive Income before Reclassifications	30	12
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(9)	14
Other Comprehensive Income (Loss), net of tax	21	26
Accumulated Other Comprehensive Income (Loss), Ending Balance	(242)	(269)	(263)
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	2	0	0
Other Comprehensive Income before Reclassifications	0	2	2
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0	0	0
Other Comprehensive Income (Loss), net of tax	0	2
Accumulated Other Comprehensive Income (Loss), Ending Balance	2	2	2
Pension and OPEB Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(398)	(386)	(386)
Other Comprehensive Income before Reclassifications	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	6	8
Other Comprehensive Income (Loss), net of tax	6	8
Accumulated Other Comprehensive Income (Loss), Ending Balance	(392)	(378)	(398)
Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	133	91	91
Other Comprehensive Income before Reclassifications	30	10
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(15)	6
Other Comprehensive Income (Loss), net of tax	15	16
Accumulated Other Comprehensive Income (Loss), Ending Balance	148	107	133
Power [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(211)	(240)	(240)
Other Comprehensive Income before Reclassifications	28	10
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(4)	13
Other Comprehensive Income (Loss), net of tax	24	23
Accumulated Other Comprehensive Income (Loss), Ending Balance	(187)	(217)	(211)
Power [Member] | Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	0	0	0
Other Comprehensive Income before Reclassifications	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0	0
Other Comprehensive Income (Loss), net of tax	0	0
Accumulated Other Comprehensive Income (Loss), Ending Balance	0	0	0
Power [Member] | Pension and OPEB Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(340)	(327)	(327)
Other Comprehensive Income before Reclassifications	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	5	7
Other Comprehensive Income (Loss), net of tax	5	7
Accumulated Other Comprehensive Income (Loss), Ending Balance	(335)	(320)	(340)
Power [Member] | Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	129	87	87
Other Comprehensive Income before Reclassifications	28	10
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(9)	6
Other Comprehensive Income (Loss), net of tax	19	16
Accumulated Other Comprehensive Income (Loss), Ending Balance	$ 148	$ 103	$ 129