Financial Instruments (Effect of Cash Flow Hedges on Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Flow Hedging [Member]
Derivative [Line Items]
Beginning balance, after-tax	$ 7	$ (24)	$ (6)
Additions and revaluations of derivatives designated as cash flow hedges, after-tax		20	(25)
Clearance of hedge results to earnings, after-tax		11	7
Ending balance, after-tax		$ 7	$ (24)
Scenario, Forecast [Member]
Derivative [Line Items]
Portion of ending balance of gain (loss) expected to be reclassified into earnings over the next twelve months, after-tax	$ (10)