Financial Instruments - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Gross unrealized gains or losses for cash equivalent and marketable securities	$ 0	$ 0
Securities that were in an unrealized loss position for more than 12 months.	0	0
Foreign currency forward contracts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Unrealized losses related to effective portion of changes in fair value of foreign currency forward contracts	900,000
Reclassification from accumulated other comprehensive loss into earnings over next 12 month	1,000,000
Notional value	$ 139,700,000	$ 79,600,000
Maximum | Foreign currency forward contracts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Foreign currency forward contract maturity	15 months