Comprehensive Income (Loss) (Reclassification Out of AOCI) (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other comprehensive income, Recognition of net actuarial loss for pension settlement costs, before tax		$ 43
Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension Benefits [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other comprehensive income, Recognition of net actuarial loss for pension settlement costs, before tax	[1],[2]	(43)
Amounts reclassified from AOCI, before tax	[1],[2]	(51)	$ (11)
Reclassification out of Accumulated Other Comprehensive Income [Member] | 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]
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss), Net Actuarial Gain (Loss), before Tax	[1],[2]	(8)	(11)
Reclassification out of Accumulated Other Comprehensive Income [Member] | 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]	19	4
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[1],[2]	(32)	(7)
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]
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss), Net Prior Service Cost (Credit), before Tax	[1],[2]	2	2
Reclassification out of Accumulated Other Comprehensive Income [Member] | 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	2
Amounts reclassified from AOCI, tax	[1],[2]	(1)	(1)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[1],[2]	$ 1	$ 1

[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 15 - Retirement Plans for additional details).