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Investments - Available-for-sale Securities (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]    
Amortized Costs $ 8,954 $ 48,735
Gross Unrealized Holding Gains 0 89
Gross Unrealized Holding Losses (1) (6)
Fair Value 8,953 48,818
Commercial paper    
Debt Securities, Available-for-sale [Line Items]    
Amortized Costs 6,498 15,992
Gross Unrealized Holding Gains 0 2
Gross Unrealized Holding Losses (1) (3)
Fair Value 6,497 15,991
Corporate notes/bonds    
Debt Securities, Available-for-sale [Line Items]    
Amortized Costs 1,205 29,227
Gross Unrealized Holding Gains 0 72
Gross Unrealized Holding Losses 0 (3)
Fair Value 1,205 29,296
U.S. Treasuries    
Debt Securities, Available-for-sale [Line Items]    
Amortized Costs   2,251
Gross Unrealized Holding Gains   2
Gross Unrealized Holding Losses   0
Fair Value   2,253
U.S. Government agency securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Costs 1,251 1,265
Gross Unrealized Holding Gains 0 13
Gross Unrealized Holding Losses 0 0
Fair Value $ 1,251 $ 1,278