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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
Jun. 30, 2019
Dec. 31, 2018
Assets    
Total assets measured at fair value $ 70,844 $ 106,854
Liabilities    
Financing derivative   16
Total liabilities measured at fair value   16
Cash and cash equivalents [Member]    
Assets    
Total cash and cash equivalents 30,358 18,844
Cash and money market funds [Member]    
Assets    
Total cash and cash equivalents 17,124 18,844
Commercial paper [Member]    
Assets    
Total cash and cash equivalents 10,386  
U.S. Treasury securities [Member]    
Assets    
Total cash and cash equivalents 2,848  
Investments [Member]    
Assets    
Total investments 36,486 83,510
Commercial paper, not included with cash and cash equivalents [Member]    
Assets    
Total investments 29,592 53,469
Corporate debt securities [Member]    
Assets    
Total investments 5,497 10,214
U.S. government and agency securities, not included with cash and cash equivalents [Member]    
Assets    
Total investments 1,397 19,827
Certified deposits [Member]    
Assets    
Long-term restricted cash 4,000 4,500
Level 1 [Member]    
Assets    
Total assets measured at fair value 21,124 23,344
Level 1 [Member] | Cash and cash equivalents [Member]    
Assets    
Total cash and cash equivalents 17,124 18,844
Level 1 [Member] | Cash and money market funds [Member]    
Assets    
Total cash and cash equivalents 17,124 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 49,720 83,510
Level 2 [Member] | Cash and cash equivalents [Member]    
Assets    
Total cash and cash equivalents 13,234  
Level 2 [Member] | Commercial paper [Member]    
Assets    
Total cash and cash equivalents 10,386  
Level 2 [Member] | U.S. Treasury securities [Member]    
Assets    
Total cash and cash equivalents 2,848  
Level 2 [Member] | Investments [Member]    
Assets    
Total investments 36,486 83,510
Level 2 [Member] | Commercial paper, not included with cash and cash equivalents [Member]    
Assets    
Total investments 29,592 53,469
Level 2 [Member] | Corporate debt securities [Member]    
Assets    
Total investments 5,497 10,214
Level 2 [Member] | U.S. government and agency securities, not included with cash and cash equivalents [Member]    
Assets    
Total investments $ 1,397 19,827
Level 3 [Member]    
Liabilities    
Financing derivative   16
Total liabilities measured at fair value   $ 16