Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements - Fair Value of Financial Assets and Liabilities Measured on Recurring Basis (Details)

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Fair Value Measurements - Fair Value of Financial Assets and Liabilities Measured on Recurring Basis (Details) - Fair Value Measurements Recurring - USD ($)
Sep. 30, 2021
Dec. 31, 2020
Assets:    
Total $ 293,381,000 $ 366,617,000
Level 1    
Assets:    
Total 107,502,000 286,061,000
Level 2    
Assets:    
Total 185,879,000 80,556,000
Level 3    
Assets:    
Total 0 0
Money Market Funds    
Assets:    
Total 30,108,000 204,632,000
Money Market Funds | Level 1    
Assets:    
Total 30,108,000 204,632,000
Commercial Paper    
Assets:    
Total 56,568,000 42,208,000
Commercial Paper | Level 2    
Assets:    
Total 56,568,000 42,208,000
Corporate Debt Securities    
Assets:    
Total 70,276,000 25,716,000
Corporate Debt Securities | Level 2    
Assets:    
Total 70,276,000 25,716,000
Equity Securities    
Assets:    
Total 39,763,000 41,644,000
Equity Securities | Level 1    
Assets:    
Total 39,763,000 41,644,000
Asset-backed Securities    
Assets:    
Total 37,595,000 12,632,000
Asset-backed Securities | Level 2    
Assets:    
Total 37,595,000 12,632,000
U.S. Government Securities    
Assets:    
Total 37,631,000 39,785,000
U.S. Government Securities | Level 1    
Assets:    
Total 37,631,000 $ 39,785,000
Supranational Debt Securities    
Assets:    
Total 21,440,000  
Supranational Debt Securities | Level 2    
Assets:    
Total $ 21,440,000