Comprehensive Income (Loss) (Reclassification Out of AOCI) (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated Defined Benefit Plans Adjustment [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI, net of tax				$ (11)	$ (11)
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]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	[1],[2]	$ 9	$ 10	17	21
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]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	[1],[2]	19		62
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]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	[1],[2]	(28)	(10)	(79)	(21)
Amounts reclassified from AOCI, tax	[1],[2]	10	4	29	8
Amounts reclassified from AOCI, net of tax	[1],[2]	(18)	(6)	(50)	(13)
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]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	[1],[2]	3	3	5	5
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]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	[1],[2]	1	1	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]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	[1],[2]	2	2	4	4
Amounts reclassified from AOCI, tax	[1],[2]		(1)	(1)	(2)
Amounts reclassified from AOCI, net of tax	[1],[2]	$ 2	$ 1	$ 3	$ 2

[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).