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Short-Term Investments (Details) - USD ($)
$ in Thousands
Jun. 30, 2021
Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost $ 120,211 $ 29,528
Gross Unrealized Losses (3)  
Estimated Fair Value 120,208 29,528
Municipal bonds and notes    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 2,027 28,998
Gross Unrealized Losses (1)  
Estimated Fair Value 2,026 28,998
Zero coupon bonds    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 117,654  
Gross Unrealized Losses (2)  
Estimated Fair Value 117,652  
Certificates of deposit    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 530 530
Estimated Fair Value $ 530 $ 530