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Fair Value of Financial Instruments (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Financial assets subject to fair value measurements and the necessary disclosures    
Fair value, available-for-sale securities $ 352,015 $ 441,186
Money market funds    
Financial assets subject to fair value measurements and the necessary disclosures    
Fair value, available-for-sale securities 84,158 18,162
US Government bonds and notes    
Financial assets subject to fair value measurements and the necessary disclosures    
Fair value, available-for-sale securities 22,052 169,633
Corporate notes, bonds and commercial paper    
Financial assets subject to fair value measurements and the necessary disclosures    
Fair value, available-for-sale securities 245,805 253,391
Recurring basis    
Financial assets subject to fair value measurements and the necessary disclosures    
Fair value, available-for-sale securities 352,015 441,186
Recurring basis | Money market funds    
Financial assets subject to fair value measurements and the necessary disclosures    
Fair value, available-for-sale securities 84,158 18,162
Recurring basis | US Government bonds and notes    
Financial assets subject to fair value measurements and the necessary disclosures    
Fair value, available-for-sale securities 22,052 169,633
Recurring basis | Corporate notes, bonds and commercial paper    
Financial assets subject to fair value measurements and the necessary disclosures    
Fair value, available-for-sale securities 245,805 253,391
Recurring basis | Quoted market prices in active markets (Level 1)    
Financial assets subject to fair value measurements and the necessary disclosures    
Fair value, available-for-sale securities 84,158 18,162
Recurring basis | Quoted market prices in active markets (Level 1) | Money market funds    
Financial assets subject to fair value measurements and the necessary disclosures    
Fair value, available-for-sale securities 84,158 18,162
Recurring basis | Quoted market prices in active markets (Level 1) | US Government bonds and notes    
Financial assets subject to fair value measurements and the necessary disclosures    
Fair value, available-for-sale securities 0 0
Recurring basis | Quoted market prices in active markets (Level 1) | Corporate notes, bonds and commercial paper    
Financial assets subject to fair value measurements and the necessary disclosures    
Fair value, available-for-sale securities 0 0
Recurring basis | Significant other observable inputs (Level 2)    
Financial assets subject to fair value measurements and the necessary disclosures    
Fair value, available-for-sale securities 267,857 423,024
Recurring basis | Significant other observable inputs (Level 2) | Money market funds    
Financial assets subject to fair value measurements and the necessary disclosures    
Fair value, available-for-sale securities 0 0
Recurring basis | Significant other observable inputs (Level 2) | US Government bonds and notes    
Financial assets subject to fair value measurements and the necessary disclosures    
Fair value, available-for-sale securities 22,052 169,633
Recurring basis | Significant other observable inputs (Level 2) | Corporate notes, bonds and commercial paper    
Financial assets subject to fair value measurements and the necessary disclosures    
Fair value, available-for-sale securities 245,805 253,391
Recurring basis | Significant unobservable inputs (Level 3)    
Financial assets subject to fair value measurements and the necessary disclosures    
Fair value, available-for-sale securities 0 0
Recurring basis | Significant unobservable inputs (Level 3) | Money market funds    
Financial assets subject to fair value measurements and the necessary disclosures    
Fair value, available-for-sale securities 0 0
Recurring basis | Significant unobservable inputs (Level 3) | US Government bonds and notes    
Financial assets subject to fair value measurements and the necessary disclosures    
Fair value, available-for-sale securities 0 0
Recurring basis | Significant unobservable inputs (Level 3) | Corporate notes, bonds and commercial paper    
Financial assets subject to fair value measurements and the necessary disclosures    
Fair value, available-for-sale securities $ 0 $ 0