Comprehensive Income (Loss) (Reclassification Out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Costs/Benefits [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI, net of tax			$ (38)	$ (63)
OPEB Costs [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI, net of tax			4	4
Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension Costs/Benefits [Member] | Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI, before tax	$ (4)	$ (6)	(17)	(23)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension Costs/Benefits [Member] | Accumulated Defined Benefit Plans Adjustment, Net Actuarial Loss for Pension Settlement Costs [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI, before tax	(9)	(15)	(34)	(77)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension Costs/Benefits [Member] | Accumulated Defined Benefit Plans Adjustment [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI, before tax	(13)	(21)	(51)	(100)
Amounts reclassified from AOCI, tax	2	8	13	37
Amounts reclassified from AOCI, net of tax	(11)	(13)	(38)	(63)
Reclassification out of Accumulated Other Comprehensive Income [Member] | OPEB Costs [Member] | Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI, before tax	1	2	6	7
Reclassification out of Accumulated Other Comprehensive Income [Member] | OPEB Costs [Member] | Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI, before tax		1	(1)
Reclassification out of Accumulated Other Comprehensive Income [Member] | OPEB Costs [Member] | Accumulated Defined Benefit Plans Adjustment [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI, before tax	1	3	5	7
Amounts reclassified from AOCI, tax		(2)	(1)	(3)
Amounts reclassified from AOCI, net of tax	$ 1	$ 1	$ 4	$ 4