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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
Sep. 30, 2019
Dec. 31, 2018
Assets    
Total assets measured at fair value $ 53,204 $ 106,854
Liabilities    
Financing derivative   16
Total liabilities measured at fair value   16
Cash and cash equivalents [Member]    
Assets    
Total cash and cash equivalents 33,084 18,844
Cash and money market funds [Member]    
Assets    
Total cash and cash equivalents 18,505 18,844
Commercial paper [Member]    
Assets    
Total cash and cash equivalents 14,579  
Investments [Member]    
Assets    
Total investments 16,120 83,510
Commercial paper, not included with cash and cash equivalents [Member]    
Assets    
Total investments 12,118 53,469
Corporate debt securities [Member]    
Assets    
Total investments 4,002 10,214
U.S. government and agency securities, not included with cash and cash equivalents [Member]    
Assets    
Total investments   19,827
Certified deposits [Member]    
Assets    
Long-term restricted cash 4,000 4,500
Level 1 [Member]    
Assets    
Total assets measured at fair value 22,505 23,344
Level 1 [Member] | Cash and cash equivalents [Member]    
Assets    
Total cash and cash equivalents 18,505 18,844
Level 1 [Member] | Cash and money market funds [Member]    
Assets    
Total cash and cash equivalents 18,505 18,844
Level 1 [Member] | Certified deposits [Member]    
Assets    
Long-term restricted cash 4,000 4,500
Level 2 [Member]    
Assets    
Total assets measured at fair value 30,699 83,510
Level 2 [Member] | Cash and cash equivalents [Member]    
Assets    
Total cash and cash equivalents 14,579  
Level 2 [Member] | Commercial paper [Member]    
Assets    
Total cash and cash equivalents 14,579  
Level 2 [Member] | Investments [Member]    
Assets    
Total investments 16,120 83,510
Level 2 [Member] | Commercial paper, not included with cash and cash equivalents [Member]    
Assets    
Total investments 12,118 53,469
Level 2 [Member] | Corporate debt securities [Member]    
Assets    
Total investments $ 4,002 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
Total liabilities measured at fair value   $ 16