Financial Instruments - Additional Information (Details)	9 Months Ended
	Sep. 30, 2018 USD ($)	Dec. 31, 2017 Investment
Investments Debt And Equity Securities [Abstract]
Remaining contractual maturities of available-for-sale-securities	less than one year
Realized gains (losses) on available-for-sale securities	$ 0
Unrealized losses on cash equivalents and investments	173,000
Gross unrealized loss in continuous loss position for 12 months or longer	100,000
Number of investments in continuous loss position for 12 months or longer | Investment		0
Cash, cash equivalents, restricted cash and investments	141,900,000
Cash portion included in cash, cash equivalents, restricted cash and investments	1,300,000
Cash equivalents, restricted cash and investments	$ 140,600,000