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Fair Value Measurement (Details) - USD ($)
$ in Thousands
Jun. 30, 2021
Dec. 31, 2020
Level 2    
Assets:    
Available-for-sale investments $ 896,862 $ 1,003,348
Recurring    
Assets:    
Total assets 1,035,151 1,182,650
Liabilities:    
Development derivative liability 11,607 0
Recurring | Corporate notes and bonds    
Assets:    
Available-for-sale investments 403,551 687,469
Recurring | Corporate commercial paper    
Assets:    
Available-for-sale investments 487,430 313,497
Recurring | Obligations of U.S. government agencies    
Assets:    
Available-for-sale investments 5,881 2,382
Recurring | Money market funds    
Assets:    
Money market funds 138,289 179,302
Recurring | Level 1    
Assets:    
Total assets 138,289 179,302
Liabilities:    
Development derivative liability 0 0
Recurring | Level 1 | Corporate notes and bonds    
Assets:    
Available-for-sale investments 0 0
Recurring | Level 1 | Corporate commercial paper    
Assets:    
Available-for-sale investments 0 0
Recurring | Level 1 | Obligations of U.S. government agencies    
Assets:    
Available-for-sale investments 0 0
Recurring | Level 1 | Money market funds    
Assets:    
Money market funds 138,289 179,302
Recurring | Level 2    
Assets:    
Total assets 896,862 1,003,348
Liabilities:    
Development derivative liability 0 0
Recurring | Level 2 | Corporate notes and bonds    
Assets:    
Available-for-sale investments 403,551 687,469
Recurring | Level 2 | Corporate commercial paper    
Assets:    
Available-for-sale investments 487,430 313,497
Recurring | Level 2 | Obligations of U.S. government agencies    
Assets:    
Available-for-sale investments 5,881 2,382
Recurring | Level 2 | Money market funds    
Assets:    
Money market funds 0 0
Recurring | Level 3    
Assets:    
Total assets 0 0
Liabilities:    
Development derivative liability 11,607 0
Recurring | Level 3 | Corporate notes and bonds    
Assets:    
Available-for-sale investments 0 0
Recurring | Level 3 | Corporate commercial paper    
Assets:    
Available-for-sale investments 0 0
Recurring | Level 3 | Obligations of U.S. government agencies    
Assets:    
Available-for-sale investments 0 0
Recurring | Level 3 | Money market funds    
Assets:    
Money market funds $ 0 $ 0