Accumulated Other Comprehensive Income (Loss), Net of Tax (Reclassifications of AOCI) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Jun. 30, 2014	Sep. 30, 2014	Sep. 30, 2013
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	$ 28	$ 11		$ 51	$ 17
Reclassification from Accumulated Other Comprehensive Income, Current Period, Tax	(15)	(7)		(30)	(13)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	13	4		21	4
Cash Flow Hedges [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Energy-Related Contracts, Pre-Tax	(1)	(3)	12	11	(10)
Energy-Related Contracts, Tax	0	(1)		5	(4)
Energy-Related Contracts, After-Tax	(1)	(2)		6	(6)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	1	2	(7)	(6)	6
Pension and OPEB Plans [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification Adjustment from AOCI, Pension and OPEB, Pre-Tax	(5)	(15)		(14)	(48)
Reclassification Adjustment from AOCI, Pension and OPEB, Tax	2	6		5	20
Reclassification Adjustment from AOCI, Pension and OPEB, After-Tax	(3)	(9)		(9)	(28)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(3)	(9)		(9)	(28)
Available-for-Sale Securities [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification for Available for Sale Securities, Pre-Tax	32	23		76	55
Reclassification for Available for Sale Securities, Tax	(17)	(12)		(40)	(29)
Reclassification for Available for Sale Securities, After-Tax	15	11		36	26
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	15	11		36	26
Power [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	31	12		53	23
Reclassification from Accumulated Other Comprehensive Income, Current Period, Tax	(16)	(7)		(30)	(15)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	15	5		23	8
Power [Member] | Cash Flow Hedges [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Energy-Related Contracts, Pre-Tax	(1)	(3)		11	(11)
Energy-Related Contracts, Tax	0	(1)		5	(4)
Energy-Related Contracts, After-Tax	(1)	(2)		6	(7)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	1	2		(6)	7
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	(3)	(14)		(11)	(42)
Reclassification Adjustment from AOCI, Pension and OPEB, Tax	1	6		4	17
Reclassification Adjustment from AOCI, Pension and OPEB, After-Tax	(2)	(8)		(7)	(25)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(2)	(8)		(7)	(25)
Power [Member] | Available-for-Sale Securities [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification for Available for Sale Securities, Pre-Tax	33	23		75	54
Reclassification for Available for Sale Securities, Tax	(17)	(12)		(39)	(28)
Reclassification for Available for Sale Securities, After-Tax	16	11		36	26
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	16	11		36	26
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		8	8
Amortization of Prior Service (Cost) Credit, Tax	(1)	(1)		(3)	(3)
Amortization of Prior Service (Cost) Credit, After-Tax	2	2		5	5
Amortization of Actuarial Loss, Pre-Tax	(8)	(18)		(22)	(56)
Amortization of Actuarial Loss, Tax	3	7		8	23
Amortization of Actuarial Loss, After-Tax	(5)	(11)		(14)	(33)
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		7	7
Amortization of Prior Service (Cost) Credit, Tax	(1)	(1)		(3)	(3)
Amortization of Prior Service (Cost) Credit, After-Tax	2	2		4	4
Amortization of Actuarial Loss, Pre-Tax	(6)	(17)		18	49
Amortization of Actuarial Loss, Tax	2	7		(7)	(20)
Amortization of Actuarial Loss, After-Tax	(4)	(10)		11	29
Operating Revenues [Member] | Cash Flow Hedges [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Energy-Related Contracts, Pre-Tax	(1)	3		11	(11)
Energy-Related Contracts, Tax	0	1		5	(4)
Energy-Related Contracts, After-Tax	(1)	2		6	(7)
Operating Revenues [Member] | Power [Member] | Cash Flow Hedges [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Energy-Related Contracts, Pre-Tax	(1)	(3)		11	(11)
Energy-Related Contracts, Tax	0	(1)		5	(4)
Energy-Related Contracts, After-Tax	(1)	(2)		6	(7)
Interest Expense [Member] | Cash Flow Hedges [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Energy-Related Contracts, Pre-Tax	0	0		0	1
Energy-Related Contracts, Tax	0	0			0
Energy-Related Contracts, After-Tax	0	0		0	1
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	47	35		105	99
Reclassification for Available for Sale Securities, Tax	(24)	(18)		(54)	(51)
Reclassification for Available for Sale Securities, After-Tax	23	17		51	48
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	45	35		101	95
Reclassification for Available for Sale Securities, Tax	(23)	(18)		(52)	(49)
Reclassification for Available for Sale Securities, After-Tax	22	17		49	46
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	(5)	(9)		(15)	(37)
Reclassification for Available for Sale Securities, Tax	2	4		7	18
Reclassification for Available for Sale Securities, After-Tax	(3)	(5)		(8)	(19)
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	(2)	(9)		(12)	(34)
Reclassification for Available for Sale Securities, Tax	1	4		6	17
Reclassification for Available for Sale Securities, After-Tax	(1)	(5)		(6)	(17)
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	(10)	(3)		(14)	(7)
Reclassification for Available for Sale Securities, Tax	5	2		7	4
Reclassification for Available for Sale Securities, After-Tax	(5)	(1)		(7)	(3)
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	(10)	(3)		(14)	(7)
Reclassification for Available for Sale Securities, Tax	5	2		7	4
Reclassification for Available for Sale Securities, After-Tax	$ (5)	$ (1)		$ (7)	$ (3)