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Fair Value Measurements (Details) - Recurring - USD ($)
$ in Thousands
Jun. 30, 2021
Dec. 31, 2020
Assets:    
Assets, fair value $ 170,964 $ 187,817
Liabilities:    
Liabilities, fair value 663 1,376
Derivative liability for Exit Fee    
Liabilities:    
Liabilities, fair value 663 1,376
Money market funds    
Assets:    
Assets, fair value 84,650 88,151
Commercial paper    
Assets:    
Assets, fair value 63,975 62,729
Corporate bonds    
Assets:    
Assets, fair value 16,964 26,658
Asset-backed securities    
Assets:    
Assets, fair value 5,375  
U.S. government-sponsored agency bonds    
Assets:    
Assets, fair value   9,279
U.S. treasury notes    
Assets:    
Assets, fair value   1,000
Level 1    
Assets:    
Assets, fair value 84,650 88,151
Liabilities:    
Liabilities, fair value 0 0
Level 1 | Derivative liability for Exit Fee    
Liabilities:    
Liabilities, fair value 0 0
Level 1 | Money market funds    
Assets:    
Assets, fair value 84,650 88,151
Level 1 | Commercial paper    
Assets:    
Assets, fair value 0 0
Level 1 | Corporate bonds    
Assets:    
Assets, fair value 0 0
Level 1 | Asset-backed securities    
Assets:    
Assets, fair value 0  
Level 1 | U.S. government-sponsored agency bonds    
Assets:    
Assets, fair value   0
Level 1 | U.S. treasury notes    
Assets:    
Assets, fair value   0
Level 2    
Assets:    
Assets, fair value 86,314 99,666
Liabilities:    
Liabilities, fair value 0 0
Level 2 | Derivative liability for Exit Fee    
Liabilities:    
Liabilities, fair value 0 0
Level 2 | Money market funds    
Assets:    
Assets, fair value 0 0
Level 2 | Commercial paper    
Assets:    
Assets, fair value 63,975 62,729
Level 2 | Corporate bonds    
Assets:    
Assets, fair value 16,964 26,658
Level 2 | Asset-backed securities    
Assets:    
Assets, fair value 5,375  
Level 2 | U.S. government-sponsored agency bonds    
Assets:    
Assets, fair value   9,279
Level 2 | U.S. treasury notes    
Assets:    
Assets, fair value   1,000
Level 3    
Assets:    
Assets, fair value 0 0
Liabilities:    
Liabilities, fair value 663 1,376
Level 3 | Derivative liability for Exit Fee    
Liabilities:    
Liabilities, fair value 663 1,376
Level 3 | Money market funds    
Assets:    
Assets, fair value 0 0
Level 3 | Commercial paper    
Assets:    
Assets, fair value 0 0
Level 3 | Corporate bonds    
Assets:    
Assets, fair value 0 0
Level 3 | Asset-backed securities    
Assets:    
Assets, fair value $ 0  
Level 3 | U.S. government-sponsored agency bonds    
Assets:    
Assets, fair value   0
Level 3 | U.S. treasury notes    
Assets:    
Assets, fair value   $ 0