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Summary of Significant Accounting Policies (Summary of Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($)
Mar. 31, 2020
Feb. 29, 2020
Dec. 31, 2019
Assets      
Total cash and cash equivalents $ 68,862,000   $ 29,627,000
Total investments 73,729,000   19,472,000
Long-term restricted cash 4,000,000   4,000,000
Total assets measured at fair value 146,591,000   53,099,000
Liabilities      
Financing derivative $ 0
Continuation Advances  
Total liabilities measured at fair value  
Cash and money market funds [Member]      
Assets      
Total cash and cash equivalents 32,529,000   18,644,000
Commercial paper [Member]      
Assets      
Total cash and cash equivalents 36,333,000   10,983,000
Commercial paper, not included with cash and cash equivalents [Member]      
Assets      
Total investments 45,710,000   16,971,000
Corporate debt securities [Member]      
Assets      
Total investments 12,924,000   2,501,000
U.S. government & agency securities, not included with cash and cash equivalents [Member]      
Assets      
Total investments 15,095,000    
Level 1 [Member]      
Assets      
Total cash and cash equivalents 32,529,000   18,644,000
Long-term restricted cash 4,000,000   4,000,000
Total assets measured at fair value 36,529,000   22,644,000
Liabilities      
Financing derivative  
Continuation Advances  
Total liabilities measured at fair value  
Level 1 [Member] | Cash and money market funds [Member]      
Assets      
Total cash and cash equivalents 32,529,000   18,644,000
Level 2 [Member]      
Assets      
Total cash and cash equivalents 36,333,000   10,983,000
Total investments 73,729,000   19,472,000
Total assets measured at fair value 110,062,000   30,455,000
Liabilities      
Financing derivative  
Continuation Advances  
Total liabilities measured at fair value  
Level 2 [Member] | Commercial paper [Member]      
Assets      
Total cash and cash equivalents 36,333,000   10,983,000
Level 2 [Member] | Commercial paper, not included with cash and cash equivalents [Member]      
Assets      
Total investments 45,710,000   16,971,000
Level 2 [Member] | Corporate debt securities [Member]      
Assets      
Total investments 12,924,000   2,501,000
Level 2 [Member] | U.S. government & agency securities, not included with cash and cash equivalents [Member]      
Assets      
Total investments 15,095,000    
Level 3 [Member]      
Liabilities      
Financing derivative  
Continuation Advances  
Total liabilities measured at fair value