Accumulated Other Comprehensive Income (Loss), Net of Tax (Reclassifications of AOCI) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	$ 18	$ (25)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Tax	(9)	11
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	9	(14)
Cash Flow Hedges [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0	0	$ 0
Pension and OPEB Plans [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification Adjustment from AOCI, Pension and OPEB, Pre-Tax	(10)	(14)
Reclassification Adjustment from AOCI, Pension and OPEB, Tax	4	6
Reclassification Adjustment from AOCI, Pension and OPEB, After-Tax	(6)	(8)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(6)	(8)
Available-for-Sale Securities [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification for Available for Sale Securities, Pre-Tax	28	(11)
Reclassification for Available for Sale Securities, Tax	(13)	5
Reclassification for Available for Sale Securities, After-Tax	15	(6)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	15	(6)
Power [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	10	(23)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Tax	(6)	10
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	4	(13)
Power [Member] | Cash Flow Hedges [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0	0
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	(9)	(12)
Reclassification Adjustment from AOCI, Pension and OPEB, Tax	4	5
Reclassification Adjustment from AOCI, Pension and OPEB, After-Tax	(5)	(7)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(5)	(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	19	(11)
Reclassification for Available for Sale Securities, Tax	(10)	5
Reclassification for Available for Sale Securities, After-Tax	9	(6)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	9	(6)
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	2	3
Amortization of Prior Service (Cost) Credit, Tax	(1)	(1)
Amortization of Prior Service (Cost) Credit, After-Tax	1	2
Amortization of Actuarial Loss, Pre-Tax	(12)	(17)
Amortization of Actuarial Loss, Tax	5	7
Amortization of Actuarial Loss, After-Tax	(7)	(10)
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	2	3
Amortization of Prior Service (Cost) Credit, Tax	(1)	(1)
Amortization of Prior Service (Cost) Credit, After-Tax	1	2
Amortization of Actuarial Loss, Pre-Tax	(11)	(15)
Amortization of Actuarial Loss, Tax	5	6
Amortization of Actuarial Loss, After-Tax	(6)	(9)
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	36	16
Reclassification for Available for Sale Securities, Tax	(17)	(8)
Reclassification for Available for Sale Securities, After-Tax	19	8
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	25	15
Reclassification for Available for Sale Securities, Tax	(13)	(8)
Reclassification for Available for Sale Securities, After-Tax	12	7
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	(7)	(17)
Reclassification for Available for Sale Securities, Tax	3	8
Reclassification for Available for Sale Securities, After-Tax	(4)	(9)
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	(5)	(16)
Reclassification for Available for Sale Securities, Tax	2	8
Reclassification for Available for Sale Securities, After-Tax	(3)	(8)
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	(1)	(10)
Reclassification for Available for Sale Securities, Tax	1	5
Reclassification for Available for Sale Securities, After-Tax	0	(5)
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	(1)	(10)
Reclassification for Available for Sale Securities, Tax	1	5
Reclassification for Available for Sale Securities, After-Tax	$ 0	$ (5)