Accumulated Other Comprehensive Income (Loss), Net of Tax (Changes of AOCI) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Jan. 01, 2019	Jan. 01, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption				$ (81)	$ (176)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	$ (377)	$ (229)	$ (229)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	13	(16)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	4	10
Other Comprehensive Income (Loss), net of tax	17	(6)
Accumulated Other Comprehensive Income (Loss), Ending Balance	(441)	(411)	(377)
Net Change in Accumulated Other Comprehensive Income	(64)	(182)
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption				0	0
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(1)	0	0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(4)	0	(1)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0	0	0
Other Comprehensive Income (Loss), net of tax	(4)	0
Accumulated Other Comprehensive Income (Loss), Ending Balance	(5)	0	(1)
Net Change in Accumulated Other Comprehensive Income	(4)	0
Pension and OPEB Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption				(81)	0
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(360)	(406)	(406)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(3)	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	3	8
Other Comprehensive Income (Loss), net of tax	0	8
Accumulated Other Comprehensive Income (Loss), Ending Balance	(441)	(398)	(360)
Net Change in Accumulated Other Comprehensive Income	(81)	8
Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption				0	(176)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(16)	177	177
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	20	(16)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	1	2
Other Comprehensive Income (Loss), net of tax	21	(14)
Accumulated Other Comprehensive Income (Loss), Ending Balance	5	(13)	(16)
Net Change in Accumulated Other Comprehensive Income	21	(190)
Power [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption				(69)	(175)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(319)	(172)	(172)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	12	(13)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	4	8
Other Comprehensive Income (Loss), net of tax	16	(5)
Accumulated Other Comprehensive Income (Loss), Ending Balance	(372)	(352)	(319)
Net Change in Accumulated Other Comprehensive Income	(53)	(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	0
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	0	0	0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	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	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				(69)	0
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(306)	(347)	(347)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(3)	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	3	6
Other Comprehensive Income (Loss), net of tax	0	6
Accumulated Other Comprehensive Income (Loss), Ending Balance	(375)	(341)	(306)
Net Change in Accumulated Other Comprehensive Income	(69)	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				$ 0	$ (175)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(13)	175	175
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	15	(13)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	1	2
Other Comprehensive Income (Loss), net of tax	16	(11)
Accumulated Other Comprehensive Income (Loss), Ending Balance	3	(11)	$ (13)
Net Change in Accumulated Other Comprehensive Income	$ 16	$ (186)