Comprehensive Income (Loss) (Reclassification Out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Costs/Benefits [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive loss, before tax	$ 4	$ 32
Postretirement Costs [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive loss, before tax	(1)	(1)
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 accumulated other comprehensive loss, before tax	(7)	(8)
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 accumulated other comprehensive loss, before tax		(43)
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 accumulated other comprehensive loss, before tax	7	51
Amounts reclassified from AOCI, tax	3	19
Amounts reclassified from AOCI, net of tax	(4)	(32)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Postretirement 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 accumulated other comprehensive loss, before tax	(2)	(2)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Postretirement 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 accumulated other comprehensive loss, before tax	(1)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Postretirement Costs [Member] | Accumulated Defined Benefit Plans Adjustment [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive loss, before tax	(1)	(2)
Amounts reclassified from AOCI, tax		(1)
Amounts reclassified from AOCI, net of tax	$ 1	$ 1