Accumulated Other Comprehensive Income (Loss), Net of Tax Accumulated Other Comprehensive Income (Loss), Net of Tax (Changes in AOCI) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption			$ 0		$ (176)
Beginning Balance		$ (229)	(263)	$ (295)
Other Comprehensive Income before Reclassifications		(9)	77	(3)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)		37	(43)	35
Net Current Period Other Comprehensive Income (Loss)		28	34	32
Ending Balance		(377)	(229)	(263)
Net Change in Accumulated Other Comprehensive Income		(148)
Power [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption			0		(175)
Beginning Balance		(172)	(211)	(240)
Other Comprehensive Income before Reclassifications		(3)	78	(3)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)		31	(39)	32
Net Current Period Other Comprehensive Income (Loss)		28	39	29
Ending Balance		(319)	(172)	(211)
Net Change in Accumulated Other Comprehensive Income		(147)
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption					0
Beginning Balance		0	2	0
Other Comprehensive Income before Reclassifications	$ (1)	(1)	0	2
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)		0	(2)	0
Net Current Period Other Comprehensive Income (Loss)		(1)	(2)	2
Ending Balance		(1)	0	2
Net Change in Accumulated Other Comprehensive Income		(1)
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member] | Power [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption					0
Beginning Balance		0	0	0
Other Comprehensive Income before Reclassifications		0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)		0	0	0
Net Current Period Other Comprehensive Income (Loss)		0	0	0
Ending Balance		0	0	0
Net Change in Accumulated Other Comprehensive Income		0
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption					0
Beginning Balance		(406)	(398)	(386)
Other Comprehensive Income before Reclassifications		17	(32)	(45)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)		29	24	33
Net Current Period Other Comprehensive Income (Loss)		46	(8)	(12)
Ending Balance		(360)	(406)	(398)
Net Change in Accumulated Other Comprehensive Income		46
Accumulated Defined Benefit Plans Adjustment [Member] | Power [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption					0
Beginning Balance		(347)	(340)	(327)
Other Comprehensive Income before Reclassifications		16	(28)	(42)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)		25	21	29
Net Current Period Other Comprehensive Income (Loss)		41	(7)	(13)
Ending Balance		(306)	(347)	(340)
Net Change in Accumulated Other Comprehensive Income		41
Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption					(176)
Beginning Balance		177	133	91
Other Comprehensive Income before Reclassifications		(25)	109	40
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)		8	(65)	2
Net Current Period Other Comprehensive Income (Loss)		(17)	44	42
Ending Balance		(16)	177	133
Net Change in Accumulated Other Comprehensive Income		(193)
Accumulated Net Unrealized Investment Gain (Loss) [Member] | Power [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption					$ (175)
Beginning Balance		175	129	87
Other Comprehensive Income before Reclassifications		(19)	106	39
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)		6	(60)	3
Net Current Period Other Comprehensive Income (Loss)		(13)	46	42
Ending Balance		(13)	$ 175	$ 129
Net Change in Accumulated Other Comprehensive Income		$ (188)