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, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount Reclassified from Accumulated Other Comprehensive Income, Pre-Tax	$ 96	$ (62)	$ (28)
Amount Reclassified from Accumulated Other Comprehensive Income, Tax	(53)	27	10
Amount Reclassified from Accumulated Other Comprehensive Income, After-Tax	43	(35)	(18)
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	134	(6)	8
Amount Reclassified from AOCI for Available for Sale Securities, Tax	(69)	4	(6)
Amount Reclassified from AOCI for Available for Sale Securities, After-Tax	(65)	2	(2)
Amount Reclassified from Accumulated Other Comprehensive Income, After-Tax	65	(2)	2
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount Reclassified from AOCI for Pension and OPEB Plans, Pre-Tax	(41)	(56)	(56)
Amount Reclassified from AOCI for Pension and OPEB Plans, Tax	17	23	24
Amount Reclassified from AOCI for Pension and OPEB Plans, After-Tax	(24)	(33)	(32)
Amount Reclassified from Accumulated Other Comprehensive Income, After-Tax	(24)	(33)	(32)
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	3	0	20
Amount Reclassified from AOCI for Cash Flow Hedges, Tax	(1)		(8)
Amount Reclassified from AOCI for Cash Flow Hedges, After-Tax	2		12
Amount Reclassified from Accumulated Other Comprehensive Income, After-Tax	2	0	12
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
Amount Reclassified from AOCI for Cash Flow Hedges, Tax			8
Amount Reclassified from AOCI for Cash Flow Hedges, After-Tax			12
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	10	12	(12)
Amortization of Prior Service (Cost) Credit, Tax	(4)	(5)	(3)
Amortization of Prior Service (Cost) Credit, After-Tax	6	7	9
Amortization of Actuarial Loss, Pre-Tax	(51)	(68)	68
Amortization of Actuarial Loss, Tax	(21)	(28)	(27)
Amortization of Actuarial Loss, After-Tax	(30)	(40)	(41)
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	174	59	100
Amount Reclassified from AOCI for Available for Sale Securities, Tax	(89)	(29)	52
Amount Reclassified from AOCI for Available for Sale Securities, After-Tax	85	30	48
Interest Expense [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	3
Amount Reclassified from AOCI for Cash Flow Hedges, Tax	(1)
Amount Reclassified from AOCI for Cash Flow Hedges, After-Tax	2
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	(28)	(37)	(39)
Amount Reclassified from AOCI for Available for Sale Securities, Tax	14	19	(20)
Amount Reclassified from AOCI for Available for Sale Securities, After-Tax	(14)	(18)	(19)
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	(12)	(28)	(53)
Amount Reclassified from AOCI for Available for Sale Securities, Tax	6	14	(26)
Amount Reclassified from AOCI for Available for Sale Securities, After-Tax	6	14	27
Power [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount Reclassified from Accumulated Other Comprehensive Income, Pre-Tax	90	(54)	(22)
Amount Reclassified from Accumulated Other Comprehensive Income, Tax	(51)	22	7
Amount Reclassified from Accumulated Other Comprehensive Income, After-Tax	39	(32)	(15)
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	125	(6)	7
Amount Reclassified from AOCI for Available for Sale Securities, Tax	(65)	3	(6)
Amount Reclassified from AOCI for Available for Sale Securities, After-Tax	(60)	3	(1)
Amount Reclassified from Accumulated Other Comprehensive Income, After-Tax	60	(3)	1
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	(35)	(48)	(49)
Amount Reclassified from AOCI for Pension and OPEB Plans, Tax	14	19	21
Amount Reclassified from AOCI for Pension and OPEB Plans, After-Tax	(21)	(29)	(28)
Amount Reclassified from Accumulated Other Comprehensive Income, After-Tax	(21)	(29)	(28)
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
Amount Reclassified from AOCI for Cash Flow Hedges, Tax			(8)
Amount Reclassified from AOCI for Cash Flow Hedges, After-Tax			12
Amount Reclassified from Accumulated Other Comprehensive Income, After-Tax	0	0	12
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
Amount Reclassified from AOCI for Cash Flow Hedges, Tax			8
Amount Reclassified from AOCI for Cash Flow Hedges, After-Tax			12
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	9	11	(11)
Amortization of Prior Service (Cost) Credit, Tax	(4)	(5)	(3)
Amortization of Prior Service (Cost) Credit, After-Tax	5	6	8
Amortization of Actuarial Loss, Pre-Tax	(44)	(59)	60
Amortization of Actuarial Loss, Tax	(18)	(24)	(24)
Amortization of Actuarial Loss, After-Tax	(26)	(35)	(36)
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	161	55	98
Amount Reclassified from AOCI for Available for Sale Securities, Tax	(83)	(28)	51
Amount Reclassified from AOCI for Available for Sale Securities, After-Tax	78	27	47
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	(24)	(33)	(38)
Amount Reclassified from AOCI for Available for Sale Securities, Tax	12	17	(19)
Amount Reclassified from AOCI for Available for Sale Securities, After-Tax	(12)	(16)	(19)
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	(12)	(28)	(53)
Amount Reclassified from AOCI for Available for Sale Securities, Tax	6	14	(26)
Amount Reclassified from AOCI for Available for Sale Securities, After-Tax	$ 6	$ 14	$ 27