Accumulated Other Comprehensive Income (Loss), Net of Tax (Changes of AOCI) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Jan. 01, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption				$ (176)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	$ (229)	$ (263)	$ (263)
Other Comprehensive Income before Reclassifications	(16)	30
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	10	(9)
Other Comprehensive Income (Loss), net of tax	(6)	21
Accumulated Other Comprehensive Income (Loss), Ending Balance	(411)	(242)	(229)
Net Change in Accumulated Other Comprehensive Income	(182)
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption				0
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	0	2	2
Other Comprehensive Income before Reclassifications	0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0	0	(2)
Other Comprehensive Income (Loss), net of tax	0	0
Accumulated Other Comprehensive Income (Loss), Ending Balance	0	2	0
Net Change in Accumulated Other Comprehensive Income	0
Pension and OPEB Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption				0
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(406)	(398)	(398)
Other Comprehensive Income before Reclassifications	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	8	6
Other Comprehensive Income (Loss), net of tax	8	6
Accumulated Other Comprehensive Income (Loss), Ending Balance	(398)	(392)	(406)
Net Change in Accumulated Other Comprehensive Income	8
Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption				(176)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	177	133	133
Other Comprehensive Income before Reclassifications	(16)	30
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	2	(15)
Other Comprehensive Income (Loss), net of tax	(14)	15
Accumulated Other Comprehensive Income (Loss), Ending Balance	(13)	148	177
Net Change in Accumulated Other Comprehensive Income	(190)
Power [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption				(175)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(172)	(211)	(211)
Other Comprehensive Income before Reclassifications	(13)	28
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	8	(4)
Other Comprehensive Income (Loss), net of tax	(5)	24
Accumulated Other Comprehensive Income (Loss), Ending Balance	(352)	(187)	(172)
Net Change in Accumulated Other Comprehensive Income	(180)
Power [Member] | Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption				0
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
Net Change in Accumulated Other Comprehensive Income	0
Power [Member] | Pension and OPEB Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption				0
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(347)	(340)	(340)
Other Comprehensive Income before Reclassifications	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	6	5
Other Comprehensive Income (Loss), net of tax	6	5
Accumulated Other Comprehensive Income (Loss), Ending Balance	(341)	(335)	(347)
Net Change in Accumulated Other Comprehensive Income	6
Power [Member] | Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption				$ (175)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	175	129	129
Other Comprehensive Income before Reclassifications	(13)	28
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	2	(9)
Other Comprehensive Income (Loss), net of tax	(11)	19
Accumulated Other Comprehensive Income (Loss), Ending Balance	(11)	$ 148	$ 175
Net Change in Accumulated Other Comprehensive Income	$ (186)