Comprehensive Income (Loss) (Reclassification Out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Costs [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive loss, before tax				$ 23	$ 30
Postretirement Costs [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive loss, before tax				(7)	(6)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension 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],[2]	$ (6)	$ (9)	(23)	(30)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension Costs [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	[1],[2]	(15)		(77)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension 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]	21	9	100	30
Amounts reclassified from AOCI, tax	[1],[2]	8	3	37	11
Amounts reclassified from AOCI, net of tax	[1],[2]	(13)	(6)	(63)	(19)
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	[1],[2]	(2)	(2)	(7)	(7)
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],[2]	1			(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]	(3)	(2)	(7)	(6)
Amounts reclassified from AOCI, tax	[1],[2]	(2)		(3)	(2)
Amounts reclassified from AOCI, net of tax	[1],[2]	$ 1	$ 2	$ 4	$ 4

[1] Amounts in parentheses indicate losses.

[2] These accumulated other comprehensive loss components are included in the computation of net periodic pension and OPEB costs (see Note 14 - Retirement Plans for additional details).