Fair Value Measurements - Additional Information (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Long-term investments
Fair value, assets, level 1 to level 2 transfers, amount	$ 0
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value	0
Fair Value, Inputs, Level 1
Cash and cash equivalents
Cash	65,707,000	$ 49,452,000
Money market funds	57,333,000	56,640,000
Total cash and cash equivalents	123,040,000	106,092,000
Short-term investments	38,000	51,000
Long-term investments
Restricted cash, at fair value	1,181,000	1,181,000
Total cash and cash equivalents and investments	$ 124,259,000	$ 107,324,000