Comprehensive Income (Loss) (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2017	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Discount rate - used at year end to value obligation	3.80%		3.80%	4.10%
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement	$ 32
Other comprehensive income, Recognition of net actuarial loss for pension settlement costs, before tax	77
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income, Recognition Of Net Actuarial Loss For Pension Settlement Costs, Net of Tax	48
Pension Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income, Recognition of net actuarial loss for pension settlement costs, before tax	77
Other Comprehensive Income (Loss), before Reclassifications, before Tax	(34)	$ (105)
Pension Costs [Member] | Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income, Recognition of net actuarial loss for pension settlement costs, before tax	77
Other Comprehensive Income, Recognition Of Net Actuarial Loss For Pension Settlement Costs, Net of Tax	48
Postretirement Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, before Tax	$ 1