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Marketable Securities - Unrealized Gain (Loss) on Investments (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost $ 409,214 $ 149,791
Unrealized Gain 4 120
Unrealized Loss (264) (8)
Marketable securities 408,954 149,903
U.S. government securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 374,849 125,823
Unrealized Gain 1 47
Unrealized Loss (257) (6)
Marketable securities 374,593 125,864
Commercial paper    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 16,880 4,991
Unrealized Gain 0 0
Unrealized Loss 0 0
Marketable securities 16,880 4,991
Asset-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 2,017 4,294
Unrealized Gain 0 21
Unrealized Loss 0 0
Marketable securities 2,017 4,315
Corporate debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 15,468 14,683
Unrealized Gain 3 52
Unrealized Loss (7) (2)
Marketable securities $ 15,464 $ 14,733