Accumulated Other Comprehensive Income (Loss), Net of Tax Accumulated Other Comprehensive Income (Loss), Net of Tax (Changes in AOCI) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ (263)	$ (295)	$ (283)
Other Comprehensive Income before Reclassifications	77	(3)	(30)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(43)	35	18
Net Current Period Other Comprehensive Income (Loss)	34	32	(12)
Ending Balance	(229)	(263)	(295)
Power [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(211)	(240)	(228)
Other Comprehensive Income before Reclassifications	78	(3)	(27)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(39)	32	15
Net Current Period Other Comprehensive Income (Loss)	39	29	(12)
Ending Balance	(172)	(211)	(240)
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	2	0	10
Other Comprehensive Income before Reclassifications	0	2	2
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(2)	0	(12)
Net Current Period Other Comprehensive Income (Loss)	(2)	2	(10)
Ending Balance	0	2	0
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member] | Power [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	0	0	11
Other Comprehensive Income before Reclassifications	0	0	1
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0	0	(12)
Net Current Period Other Comprehensive Income (Loss)	0	0	(11)
Ending Balance	0	0	0
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(398)	(386)	(411)
Other Comprehensive Income before Reclassifications	(32)	(45)	(7)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	24	33	32
Net Current Period Other Comprehensive Income (Loss)	(8)	(12)	25
Ending Balance	(406)	(398)	(386)
Accumulated Defined Benefit Plans Adjustment [Member] | Power [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(340)	(327)	(351)
Other Comprehensive Income before Reclassifications	(28)	(42)	(4)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	21	29	28
Net Current Period Other Comprehensive Income (Loss)	(7)	(13)	24
Ending Balance	(347)	(340)	(327)
Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	133	91	118
Other Comprehensive Income before Reclassifications	109	40	(25)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(65)	2	(2)
Net Current Period Other Comprehensive Income (Loss)	44	42	(27)
Ending Balance	177	133	91
Accumulated Net Unrealized Investment Gain (Loss) [Member] | Power [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	129	87	112
Other Comprehensive Income before Reclassifications	106	39	(24)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(60)	3	(1)
Net Current Period Other Comprehensive Income (Loss)	46	42	(25)
Ending Balance	$ 175	$ 129	$ 87