Accumulated Other Comprehensive Income (Loss), Net of Tax Accumulated Other Comprehensive Income (Loss), Net of Tax (Reclassifications out of AOCI) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount Reclassified from Accumulated Other Comprehensive Income, Pre-Tax	$ (62)	$ (28)	$ 108
Amount Reclassified from Accumulated Other Comprehensive Income, Tax	27	10	(55)
Amount Reclassified from Accumulated Other Comprehensive Income, After-Tax	(35)	(18)	53
Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount Reclassified from AOCI for Available for Sale Securities, Pre-Tax	(6)	8	135
Amount Reclassified from AOCI for Available for Sale Securities, Tax	4	(6)	(66)
Amount Reclassified from AOCI for Available for Sale Securities, After-Tax	2	(2)	(69)
Amount Reclassified from Accumulated Other Comprehensive Income, After-Tax	(2)	2	69
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount Reclassified from AOCI for Pension and OPEB Plans, Pre-Tax	(56)	(56)	(18)
Amount Reclassified from AOCI for Pension and OPEB Plans, Tax	23	24	7
Amount Reclassified from AOCI for Pension and OPEB Plans, After-Tax	(33)	(32)	(11)
Amount Reclassified from Accumulated Other Comprehensive Income, After-Tax	(33)	(32)	(11)
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount Reclassified from AOCI for Cash Flow Hedges, Pre-Tax	0	20	(9)
Amount Reclassified from AOCI for Cash Flow Hedges, Tax		(8)	4
Amount Reclassified from AOCI for Cash Flow Hedges, After-Tax		12	(5)
Amount Reclassified from Accumulated Other Comprehensive Income, After-Tax	0	12	(5)
Operating Revenues [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount Reclassified from AOCI for Cash Flow Hedges, Pre-Tax		20	(9)
Amount Reclassified from AOCI for Cash Flow Hedges, Tax		(8)	(4)
Amount Reclassified from AOCI for Cash Flow Hedges, After-Tax		12	(5)
Operation and Maintenance Expense [Member] | Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of Prior Service (Cost) Credit, Pre-Tax	12	12	(10)
Amortization of Prior Service (Cost) Credit, Tax	(5)	(3)	(4)
Amortization of Prior Service (Cost) Credit, After-Tax	7	9	6
Amortization of Actuarial Loss, Pre-Tax	(68)	(68)	28
Amortization of Actuarial Loss, Tax	(28)	(27)	(11)
Amortization of Actuarial Loss, After-Tax	(40)	(41)	(17)
Other Income [Member] | Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount Reclassified from AOCI for Available for Sale Securities, Pre-Tax	59	100	181
Amount Reclassified from AOCI for Available for Sale Securities, Tax	(29)	(52)	89
Amount Reclassified from AOCI for Available for Sale Securities, After-Tax	30	48	92
Other Deductions [Member] | Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount Reclassified from AOCI for Available for Sale Securities, Pre-Tax	(37)	(39)	(26)
Amount Reclassified from AOCI for Available for Sale Securities, Tax	19	20	(13)
Amount Reclassified from AOCI for Available for Sale Securities, After-Tax	(18)	(19)	(13)
Other-Than-Temporary Impairments [Member] | Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount Reclassified from AOCI for Available for Sale Securities, Pre-Tax	(28)	(53)	(20)
Amount Reclassified from AOCI for Available for Sale Securities, Tax	14	26	(10)
Amount Reclassified from AOCI for Available for Sale Securities, After-Tax	14	27	10
Power [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount Reclassified from Accumulated Other Comprehensive Income, Pre-Tax	(54)	(22)	109
Amount Reclassified from Accumulated Other Comprehensive Income, Tax	22	7	(54)
Amount Reclassified from Accumulated Other Comprehensive Income, After-Tax	(32)	(15)	55
Power [Member] | Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount Reclassified from AOCI for Available for Sale Securities, Pre-Tax	(6)	7	134
Amount Reclassified from AOCI for Available for Sale Securities, Tax	3	(6)	(65)
Amount Reclassified from AOCI for Available for Sale Securities, After-Tax	3	(1)	(69)
Amount Reclassified from Accumulated Other Comprehensive Income, After-Tax	(3)	1	69
Power [Member] | Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount Reclassified from AOCI for Pension and OPEB Plans, Pre-Tax	(48)	(49)	(16)
Amount Reclassified from AOCI for Pension and OPEB Plans, Tax	19	21	7
Amount Reclassified from AOCI for Pension and OPEB Plans, After-Tax	(29)	(28)	(9)
Amount Reclassified from Accumulated Other Comprehensive Income, After-Tax	(29)	(28)	(9)
Power [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount Reclassified from AOCI for Cash Flow Hedges, Pre-Tax		20	(9)
Amount Reclassified from AOCI for Cash Flow Hedges, Tax		(8)	4
Amount Reclassified from AOCI for Cash Flow Hedges, After-Tax		12	(5)
Amount Reclassified from Accumulated Other Comprehensive Income, After-Tax	0	12	(5)
Power [Member] | Operating Revenues [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount Reclassified from AOCI for Cash Flow Hedges, Pre-Tax		20	(9)
Amount Reclassified from AOCI for Cash Flow Hedges, Tax		(8)	(4)
Amount Reclassified from AOCI for Cash Flow Hedges, After-Tax		12	(5)
Power [Member] | Operation and Maintenance Expense [Member] | Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of Prior Service (Cost) Credit, Pre-Tax	11	11	(9)
Amortization of Prior Service (Cost) Credit, Tax	(5)	(3)	(4)
Amortization of Prior Service (Cost) Credit, After-Tax	6	8	5
Amortization of Actuarial Loss, Pre-Tax	(59)	(60)	25
Amortization of Actuarial Loss, Tax	(24)	(24)	(11)
Amortization of Actuarial Loss, After-Tax	(35)	(36)	(14)
Power [Member] | Other Income [Member] | Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount Reclassified from AOCI for Available for Sale Securities, Pre-Tax	55	98	178
Amount Reclassified from AOCI for Available for Sale Securities, Tax	(28)	(51)	87
Amount Reclassified from AOCI for Available for Sale Securities, After-Tax	27	47	91
Power [Member] | Other Deductions [Member] | Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount Reclassified from AOCI for Available for Sale Securities, Pre-Tax	(33)	(38)	(24)
Amount Reclassified from AOCI for Available for Sale Securities, Tax	17	19	(12)
Amount Reclassified from AOCI for Available for Sale Securities, After-Tax	(16)	(19)	(12)
Power [Member] | Other-Than-Temporary Impairments [Member] | Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount Reclassified from AOCI for Available for Sale Securities, Pre-Tax	(28)	(53)	(20)
Amount Reclassified from AOCI for Available for Sale Securities, Tax	14	26	(10)
Amount Reclassified from AOCI for Available for Sale Securities, After-Tax	$ 14	$ 27	$ 10