Accumulated Other Comprehensive Income (Loss), Net of Tax (Changes of AOCI) (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	$ (269)	$ (295)	$ (270)	$ (295)	$ (283)	$ (283)
Other Comprehensive Income before Reclassifications	7		(14)	19	3
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	10		7	24	3
Other Comprehensive Income (Loss), net of tax	17		(7)	43	6
Accumulated Other Comprehensive Income (Loss), Ending Balance	(252)	(269)	(277)	(252)	(277)	(295)
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	2	0	1	0	10	10
Other Comprehensive Income before Reclassifications	(1)	2	0	1	1	2
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0	0	0	0	(10)	(12)
Other Comprehensive Income (Loss), net of tax	(1)		0	1	(9)
Accumulated Other Comprehensive Income (Loss), Ending Balance	1	2	1	1	1	0
Pension and OPEB Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(378)	(386)	(403)	(386)	(411)	(411)
Other Comprehensive Income before Reclassifications	0		0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	8		8	16	16
Other Comprehensive Income (Loss), net of tax	8		8	16	16
Accumulated Other Comprehensive Income (Loss), Ending Balance	(370)	(378)	(395)	(370)	(395)	(386)
Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	107	91	132	91	118	118
Other Comprehensive Income before Reclassifications	8		(14)	18	2
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	2		(1)	8	(3)
Other Comprehensive Income (Loss), net of tax	10		(15)	26	(1)
Accumulated Other Comprehensive Income (Loss), Ending Balance	117	107	117	117	117	91
Power [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(217)	(240)	(216)	(240)	(228)	(228)
Other Comprehensive Income before Reclassifications	6		(14)	16	3
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	10		7	23	2
Other Comprehensive Income (Loss), net of tax	16		(7)	39	5
Accumulated Other Comprehensive Income (Loss), Ending Balance	(201)	(217)	(223)	(201)	(223)	(240)
Power [Member] | Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	0	0	2	0	11	11
Other Comprehensive Income before Reclassifications	0		0	0	1
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0		0	0	(10)
Other Comprehensive Income (Loss), net of tax	0		0	0	(9)
Accumulated Other Comprehensive Income (Loss), Ending Balance	0	0	2	0	2	0
Power [Member] | Pension and OPEB Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(320)	(327)	(344)	(327)	(351)	(351)
Other Comprehensive Income before Reclassifications	0		0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	7		7	14	14
Other Comprehensive Income (Loss), net of tax	7		7	14	14
Accumulated Other Comprehensive Income (Loss), Ending Balance	(313)	(320)	(337)	(313)	(337)	(327)
Power [Member] | Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	103	87	126	87	112	112
Other Comprehensive Income before Reclassifications	6		(14)	16	2
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	3		0	9	(2)
Other Comprehensive Income (Loss), net of tax	9		(14)	25	0
Accumulated Other Comprehensive Income (Loss), Ending Balance	$ 112	$ 103	$ 112	$ 112	$ 112	$ 87