Accumulated Other Comprehensive Income (Loss), Net of Tax (Reclassifications of AOCI) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Jun. 30, 2015	Sep. 30, 2015	Sep. 30, 2014
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	$ (42)	$ 28		$ (46)	$ 51
Reclassification from Accumulated Other Comprehensive Income, Current Period, Tax	18	(15)		19	(30)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(24)	13		(27)	21
Cash Flow Hedges [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Energy-Related Contracts, Pre-Tax	0	1	$ 17	17	(11)
Energy-Related Contracts, Tax	0	0		(7)	5
Energy-Related Contracts, After-Tax	0	1		10	(6)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0	1	$ 10	10	(6)
Pension and OPEB Plans [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification Adjustment from AOCI, Pension and OPEB, Pre-Tax	(14)	(5)		(42)	(14)
Reclassification Adjustment from AOCI, Pension and OPEB, Tax	5	2		17	5
Reclassification Adjustment from AOCI, Pension and OPEB, After-Tax	(9)	(3)		(25)	(9)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(9)	(3)		(25)	(9)
Available-for-Sale Securities [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification for Available for Sale Securities, Pre-Tax	(28)	32		(21)	76
Reclassification for Available for Sale Securities, Tax	13	(17)		9	(40)
Reclassification for Available for Sale Securities, After-Tax	(15)	15		(12)	36
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(15)	15		(12)	36
Power [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	(39)	31		(41)	53
Reclassification from Accumulated Other Comprehensive Income, Current Period, Tax	18	(16)		18	(30)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(21)	15		(23)	23
Power [Member] | Cash Flow Hedges [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Energy-Related Contracts, Pre-Tax	0	1		17	(11)
Energy-Related Contracts, Tax	0	0		(7)	5
Energy-Related Contracts, After-Tax	0	1		10	(6)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0	1		10	(6)
Power [Member] | Pension and OPEB Plans [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification Adjustment from AOCI, Pension and OPEB, Pre-Tax	(12)	(3)		(36)	(11)
Reclassification Adjustment from AOCI, Pension and OPEB, Tax	5	1		15	4
Reclassification Adjustment from AOCI, Pension and OPEB, After-Tax	(7)	(2)		(21)	(7)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(7)	(2)		(21)	(7)
Power [Member] | Available-for-Sale Securities [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification for Available for Sale Securities, Pre-Tax	(27)	33		(22)	75
Reclassification for Available for Sale Securities, Tax	13	(17)		10	(39)
Reclassification for Available for Sale Securities, After-Tax	(14)	16		(12)	36
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(14)	16		(12)	36
Operating Expense [Member] | Pension and OPEB Plans [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Amortization of Prior Service (Cost) Credit, Pre-Tax	3	3		9	8
Amortization of Prior Service (Cost) Credit, Tax	(1)	(1)		(3)	(3)
Amortization of Prior Service (Cost) Credit, After-Tax	2	2		6	5
Amortization of Actuarial Loss, Pre-Tax	(17)	(8)		(51)	(22)
Amortization of Actuarial Loss, Tax	6	3		20	8
Amortization of Actuarial Loss, After-Tax	(11)	(5)		(31)	(14)
Operating Expense [Member] | Power [Member] | Pension and OPEB Plans [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Amortization of Prior Service (Cost) Credit, Pre-Tax	3	3		9	7
Amortization of Prior Service (Cost) Credit, Tax	(1)	(1)		(3)	(3)
Amortization of Prior Service (Cost) Credit, After-Tax	2	2		6	4
Amortization of Actuarial Loss, Pre-Tax	(15)	(6)		(45)	(18)
Amortization of Actuarial Loss, Tax	6	2		18	7
Amortization of Actuarial Loss, After-Tax	(9)	(4)		(27)	(11)
Operating Revenues [Member] | Cash Flow Hedges [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Energy-Related Contracts, Pre-Tax	0	(1)		17	(11)
Energy-Related Contracts, Tax	0	0		(7)	5
Energy-Related Contracts, After-Tax	0	(1)		10	(6)
Operating Revenues [Member] | Power [Member] | Cash Flow Hedges [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Energy-Related Contracts, Pre-Tax	0	1		17	(11)
Energy-Related Contracts, Tax	0	0		(7)	5
Energy-Related Contracts, After-Tax	0	1		10	(6)
Other Income [Member] | Available-for-Sale Securities [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification for Available for Sale Securities, Pre-Tax	14	47		49	105
Reclassification for Available for Sale Securities, Tax	(7)	(24)		(25)	(54)
Reclassification for Available for Sale Securities, After-Tax	7	23		24	51
Other Income [Member] | Power [Member] | Available-for-Sale Securities [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification for Available for Sale Securities, Pre-Tax	14	45		47	101
Reclassification for Available for Sale Securities, Tax	(7)	(23)		(24)	(52)
Reclassification for Available for Sale Securities, After-Tax	7	22		23	49
Other Expense [Member] | Available-for-Sale Securities [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification for Available for Sale Securities, Pre-Tax	(12)	(5)		(25)	(15)
Reclassification for Available for Sale Securities, Tax	5	2		12	7
Reclassification for Available for Sale Securities, After-Tax	(7)	(3)		(13)	(8)
Other Expense [Member] | Power [Member] | Available-for-Sale Securities [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification for Available for Sale Securities, Pre-Tax	(11)	(2)		(24)	(12)
Reclassification for Available for Sale Securities, Tax	5	1		12	6
Reclassification for Available for Sale Securities, After-Tax	(6)	(1)		(12)	(6)
Other-Than-Temporary Impairments [Member] | Available-for-Sale Securities [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification for Available for Sale Securities, Pre-Tax	(30)	(10)		(45)	(14)
Reclassification for Available for Sale Securities, Tax	15	5		22	7
Reclassification for Available for Sale Securities, After-Tax	(15)	(5)		(23)	(7)
Other-Than-Temporary Impairments [Member] | Power [Member] | Available-for-Sale Securities [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification for Available for Sale Securities, Pre-Tax	(30)	(10)		(45)	(14)
Reclassification for Available for Sale Securities, Tax	15	5		22	7
Reclassification for Available for Sale Securities, After-Tax	$ (15)	$ (5)		$ (23)	$ (7)