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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, 2018
Dec. 31, 2017
Assets    
Total cash and cash equivalents $ 18,844 $ 16,507
Total investments 83,510 46,365
Long-term restricted cash 4,500 4,500
Total assets measured at fair value 106,854 67,372
Liabilities    
Financing derivative 16 183
Total liabilities measured at fair value 16 183
Cash and money market funds [Member]    
Assets    
Total cash and cash equivalents 18,844 14,858
Commercial paper [Member]    
Assets    
Total cash and cash equivalents   1,649
Commercial paper, not included with cash and cash equivalents [Member]    
Assets    
Total investments 53,469 20,394
Corporate debt securities [Member]    
Assets    
Total investments 10,214 9,034
U.S. government and agency securities, not included with cash and cash equivalents [Member]    
Assets    
Total investments 19,827 16,937
Level 1 [Member]    
Assets    
Total cash and cash equivalents 18,844 14,858
Long-term restricted cash 4,500 4,500
Total assets measured at fair value 23,344 19,358
Level 1 [Member] | Cash and money market funds [Member]    
Assets    
Total cash and cash equivalents 18,844 14,858
Level 2 [Member]    
Assets    
Total cash and cash equivalents   1,649
Total investments 83,510 46,365
Total assets measured at fair value 83,510 48,014
Level 2 [Member] | Commercial paper [Member]    
Assets    
Total cash and cash equivalents   1,649
Level 2 [Member] | Commercial paper, not included with cash and cash equivalents [Member]    
Assets    
Total investments 53,469 20,394
Level 2 [Member] | Corporate debt securities [Member]    
Assets    
Total investments 10,214 9,034
Level 2 [Member] | U.S. government and agency securities, not included with cash and cash equivalents [Member]    
Assets    
Total investments 19,827 16,937
Level 3 [Member]    
Liabilities    
Financing derivative 16 183
Total liabilities measured at fair value $ 16 $ 183