Derivative Instruments And Hedging Activities (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Oct. 01, 2017	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Notional forward foreign exchange contracts outstanding	$ 5,800	$ 4,300
Average maturity of foreign currency forward contracts, in years	2 years
Cash and equivalents	$ 1,010	2,983	$ 2,722	$ 2,334
Marketable securities held in trust	$ 185	$ 191
Maximum
Derivative Instruments, Gain (Loss) [Line Items]
Maturity of fixed-income securities, in years	5 years