Financial Instruments - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Financial instruments including money market funds maturities period		90 days		90 days
Maximum remaining maturity for short-term marketable securities		12 months		12 months
Minimum maturities of short-term marketable securities		90 days		90 days
Net gain (loss) related to forward contract	$ 1	$ 1	$ (3)
Cost method investments	$ 14	$ 14		$ 12