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Investments and Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value (Details) - Recurring - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Financial Assets    
Total $ 198,033 $ 228,216
Commercial paper    
Financial Assets    
Marketable investments 4,249 4,246
U.S. agency and government sponsored securities    
Financial Assets    
Marketable investments 4,555 7,857
U.S. states and municipalities    
Financial Assets    
Marketable investments 47,425 48,095
Corporate bonds    
Financial Assets    
Marketable investments 130,748 134,964
Money market funds    
Financial Assets    
Cash equivalents 11,056 33,054
Level 1    
Financial Assets    
Total 11,056 33,054
Level 1 | Commercial paper    
Financial Assets    
Marketable investments 0 0
Level 1 | U.S. agency and government sponsored securities    
Financial Assets    
Marketable investments 0 0
Level 1 | U.S. states and municipalities    
Financial Assets    
Marketable investments 0 0
Level 1 | Corporate bonds    
Financial Assets    
Marketable investments 0 0
Level 1 | Money market funds    
Financial Assets    
Cash equivalents 11,056 33,054
Level 2    
Financial Assets    
Total 186,977 195,162
Level 2 | Commercial paper    
Financial Assets    
Marketable investments 4,249 4,246
Level 2 | U.S. agency and government sponsored securities    
Financial Assets    
Marketable investments 4,555 7,857
Level 2 | U.S. states and municipalities    
Financial Assets    
Marketable investments 47,425 48,095
Level 2 | Corporate bonds    
Financial Assets    
Marketable investments 130,748 134,964
Level 2 | Money market funds    
Financial Assets    
Cash equivalents 0 0
Level 3    
Financial Assets    
Total 0 0
Level 3 | Commercial paper    
Financial Assets    
Marketable investments 0 0
Level 3 | U.S. agency and government sponsored securities    
Financial Assets    
Marketable investments 0 0
Level 3 | U.S. states and municipalities    
Financial Assets    
Marketable investments 0 0
Level 3 | Corporate bonds    
Financial Assets    
Marketable investments 0 0
Level 3 | Money market funds    
Financial Assets    
Cash equivalents $ 0 $ 0