Comprehensive Income (Loss) (Reclassification Out of AOCI) (Details) - Reclassification out of Accumulated Other Comprehensive Income [Member] - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension 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	[1],[2]	$ (9)	$ (9)	$ (30)	$ (22)
Pension Benefits [Member] | Amortization Of Defined Benefit Cost Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI, tax	[1],[2]	3	3	11	8
Amounts reclassified from AOCI, net of tax	[1],[2]	(6)	(6)	(19)	(14)
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 AOCI, before tax	[1],[2]	2	1	7	3
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 AOCI, before tax	[1],[2]		$ (1)	(1)	(6)
Postretirement Costs [Member] | Amortization Of Defined Benefit Cost Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI, before tax	[1],[2]	2		6	(3)
Amounts reclassified from AOCI, tax	[1],[2]			(2)	1
Amounts reclassified from AOCI, net of tax	[1],[2]	$ 2		$ 4	$ (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).