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Fair Value of Financial Instruments - Assets (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Level 2    
Assets, Fair Value Disclosure [Abstract]    
Assets, Fair Value Disclosure $ 0  
Level 3    
Assets, Fair Value Disclosure [Abstract]    
Assets, Fair Value Disclosure   $ 0
Recurring | Level 1    
Investments, Fair Value Disclosure [Abstract]    
Debt Securities, Available-for-sale 22,100 39,000
Recurring | Level 1 | Money Market Funds    
Assets, Fair Value Disclosure [Abstract]    
Cash and Cash Equivalents, Fair Value Disclosure $ 69,500 $ 66,300