Accumulated Other Comprehensive Income (Loss), Net of Tax (Changes of AOCI) (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	$ (270)	$ (283)	$ (87)	$ (283)	$ (95)
Other Comprehensive Income before Reclassifications	(14)		24	3	30
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	7		(10)	3	(8)
Other Comprehensive Income (Loss), net of tax	(7)		14	6	22
Accumulated Other Comprehensive Income (Loss), Ending Balance	(277)	(270)	(73)	(277)	(73)
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	1	10	0	10	(2)
Other Comprehensive Income before Reclassifications	0	1	1	1	(4)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0	(10)	0	(10)	7
Other Comprehensive Income (Loss), net of tax	0		1	(9)	3
Accumulated Other Comprehensive Income (Loss), Ending Balance	1	1	1	1	1
Pension and OPEB Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(403)	(411)	(234)	(411)	(238)
Other Comprehensive Income before Reclassifications	0		0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	8		2	16	6
Other Comprehensive Income (Loss), net of tax	8		2	16	6
Accumulated Other Comprehensive Income (Loss), Ending Balance	(395)	(403)	(232)	(395)	(232)
Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	132	118	147	118	145
Other Comprehensive Income before Reclassifications	(14)		23	2	34
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(1)		(12)	(3)	(21)
Other Comprehensive Income (Loss), net of tax	(15)		11	(1)	13
Accumulated Other Comprehensive Income (Loss), Ending Balance	117	132	158	117	158
Power [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(216)	(228)	(57)	(228)	(63)
Other Comprehensive Income before Reclassifications	(14)		23	3	27
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	7		(10)	2	(8)
Other Comprehensive Income (Loss), net of tax	(7)		13	5	19
Accumulated Other Comprehensive Income (Loss), Ending Balance	(223)	(216)	(44)	(223)	(44)
Power [Member] | Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	2	11	0	11	(1)
Other Comprehensive Income before Reclassifications	0		2	1	(4)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0		0	(10)	7
Other Comprehensive Income (Loss), net of tax	0		2	(9)	3
Accumulated Other Comprehensive Income (Loss), Ending Balance	2	2	2	2	2
Power [Member] | Pension and OPEB Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(344)	(351)	(201)	(351)	(204)
Other Comprehensive Income before Reclassifications	0		0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	7		2	14	5
Other Comprehensive Income (Loss), net of tax	7		2	14	5
Accumulated Other Comprehensive Income (Loss), Ending Balance	(337)	(344)	(199)	(337)	(199)
Power [Member] | Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	126	112	144	112	142
Other Comprehensive Income before Reclassifications	(14)		21	2	31
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0		(12)	(2)	(20)
Other Comprehensive Income (Loss), net of tax	(14)		9	0	11
Accumulated Other Comprehensive Income (Loss), Ending Balance	$ 112	$ 126	$ 153	$ 112	$ 153