Fair Value of Financial Instruments - Financial Assets Carried at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Noncurrent assets:
Long-term investments	$ 173,641	$ 132,188
Level 1
Noncurrent assets:
Long-term investments	0	0
Level 2
Noncurrent assets:
Long-term investments	0	0
Level 3
Noncurrent assets:
Long-term investments	173,641	132,188
Money market funds
Cash equivalents:
Money market funds	643,127	297,311
Money market funds | Level 1
Cash equivalents:
Money market funds	643,127	297,311
Money market funds | Level 2
Cash equivalents:
Money market funds	0	0
Money market funds | Level 3
Cash equivalents:
Money market funds	$ 0	$ 0