Comprehensive Income (Loss) (Reclassification Out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Costs/Benefits [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI, net of tax			$ (27)	$ (50)
OPEB Costs [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI, net of tax			3	3
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	$ (6)	$ (9)	(13)	(17)
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	(25)	(19)	(25)	(62)
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	(31)	(28)	(38)	(79)
Amounts reclassified from AOCI, tax	8	10	11	29
Amounts reclassified from AOCI, net of tax	(23)	(18)	(27)	(50)
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	3	3	5	5
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)	(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	3	2	4	4
Amounts reclassified from AOCI, tax	(1)		(1)	(1)
Amounts reclassified from AOCI, net of tax	$ 2	$ 2	$ 3	$ 3