Financial Instruments - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Financial instruments including money market funds maturities period	90 days	90 days
Maximum maturities period of long-term marketable securities	3 years	3 years
Minimum maturities period of long-term marketable securities	1 year	1 year
Maximum remaining maturity for short-term marketable securities	12 months	12 months
Minimum maturities of short-term marketable securities	90 days	90 days
Derivative instruments not designated as hedging instruments, description of terms	Derivative financial instruments are carried at fair value on our unaudited condensed consolidated balance sheets. We use foreign currency forward contracts to reduce the effects of foreign currency exchange rate fluctuations on our cash flows primarily for the Euro versus the U.S. Dollar. Our foreign currency forward contracts, which we have entered into to date, have not been designated as hedges and have had current maturities of less than 90 days
Cost method investments	$ 12	$ 12