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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
Dec. 31, 2019
Dec. 31, 2018
Assets    
Total cash and cash equivalents $ 29,627 $ 18,844
Total investments 19,472 83,510
Long-term restricted cash 4,000 4,500
Total assets measured at fair value 53,099 106,854
Liabilities    
Financing derivative   16
Continuation Advances
Total liabilities measured at fair value   16
Cash and money market funds [Member]    
Assets    
Total cash and cash equivalents 18,644 18,844
Commercial paper [Member]    
Assets    
Total cash and cash equivalents 10,983  
Commercial paper, not included with cash and cash equivalents [Member]    
Assets    
Total investments 16,971 53,469
Corporate debt securities [Member]    
Assets    
Total investments 2,501 10,214
U.S. government and agency securities, not included with cash and cash equivalents [Member]    
Assets    
Total investments   19,827
Level 1 [Member]    
Assets    
Total cash and cash equivalents 18,644 18,844
Long-term restricted cash 4,000 4,500
Total assets measured at fair value 22,644 23,344
Liabilities    
Continuation Advances
Level 1 [Member] | Cash and money market funds [Member]    
Assets    
Total cash and cash equivalents 18,644 18,844
Level 2 [Member]    
Assets    
Total cash and cash equivalents 10,983  
Total investments 19,472 83,510
Total assets measured at fair value 30,455 83,510
Liabilities    
Continuation Advances
Level 2 [Member] | Commercial paper [Member]    
Assets    
Total cash and cash equivalents 10,983  
Level 2 [Member] | Commercial paper, not included with cash and cash equivalents [Member]    
Assets    
Total investments 16,971 53,469
Level 2 [Member] | Corporate debt securities [Member]    
Assets    
Total investments 2,501 10,214
Level 2 [Member] | U.S. government and agency securities, not included with cash and cash equivalents [Member]    
Assets    
Total investments   19,827
Level 3 [Member]    
Liabilities    
Financing derivative   16
Continuation Advances
Total liabilities measured at fair value   $ 16