Comprehensive Income (Loss) (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement		$ 48
Pension Costs/Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, before Tax	$ (4)	30
Pension Costs/Benefits [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		43
Postretirement Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, before Tax