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Summary of Significant Accounting Policies (Summary of Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Assets    
Total cash and cash equivalents $ 932,398 $ 81,611
Total investments 227,921 237,203
Short-term restricted cash: Cash 836 836
Long-term restricted cash: Cash 3,500 3,500
Total assets measured at fair value 1,164,655 323,150
Liabilities    
Continuation Advances
Total liabilities measured at fair value
Cash and money market funds [Member]    
Assets    
Total cash and cash equivalents 906,453 43,040
Commercial paper [Member]    
Assets    
Total cash and cash equivalents 25,945 32,537
U.S. government and agency securities [Member]    
Assets    
Total cash and cash equivalents   170
U.S. Treasury security [Member]    
Assets    
Total cash and cash equivalents   5,864
Commercial paper, not included with cash and cash equivalents [Member]    
Assets    
Total investments 75,465 112,644
Corporate debt securities [Member]    
Assets    
Total investments 15,548 17,456
U.S. government & agency securities, not included with cash and cash equivalents [Member]    
Assets    
Total investments 136,908 107,103
Level 1 [Member]    
Assets    
Total cash and cash equivalents 906,453 43,040
Short-term restricted cash: Cash 836 836
Long-term restricted cash: Cash 3,500 3,500
Total assets measured at fair value 910,789 47,376
Liabilities    
Continuation Advances
Total liabilities measured at fair value
Level 1 [Member] | Cash and money market funds [Member]    
Assets    
Total cash and cash equivalents 906,453 43,040
Level 2 [Member]    
Assets    
Total cash and cash equivalents 25,945 38,571
Total investments 227,921 237,203
Total assets measured at fair value 253,866 275,774
Liabilities    
Continuation Advances
Total liabilities measured at fair value
Level 2 [Member] | Commercial paper [Member]    
Assets    
Total cash and cash equivalents 25,945 32,537
Level 2 [Member] | U.S. government and agency securities [Member]    
Assets    
Total cash and cash equivalents   170
Level 2 [Member] | U.S. Treasury security [Member]    
Assets    
Total cash and cash equivalents   5,864
Level 2 [Member] | Commercial paper, not included with cash and cash equivalents [Member]    
Assets    
Total investments 75,465 112,644
Level 2 [Member] | Corporate debt securities [Member]    
Assets    
Total investments 15,548 17,456
Level 2 [Member] | U.S. government & agency securities, not included with cash and cash equivalents [Member]    
Assets    
Total investments 136,908 107,103
Level 3 [Member]    
Liabilities    
Continuation Advances
Total liabilities measured at fair value