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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, 2017
Dec. 31, 2016
Assets    
Total assets measured at fair value $ 88,465 $ 76,478
Liabilities    
Total liabilities measured at fair value 170 356
Financing Derivative [Member]    
Liabilities    
Total liabilities measured at fair value 170 356
Cash and cash equivalents [Member]    
Assets    
Total cash and cash equivalents 25,931 16,765
Cash and cash equivalents [Member] | Cash and money market funds [Member]    
Assets    
Total cash and cash equivalents 22,283 14,516
Cash and cash equivalents [Member] | Commercial paper [Member]    
Assets    
Total cash and cash equivalents 3,648 2,249
Investments [Member]    
Assets    
Total investments 58,034 55,213
Investments [Member] | Commercial paper [Member]    
Assets    
Total investments 23,882 23,583
Investments [Member] | Corporate debt securities [Member]    
Assets    
Total investments 9,399 10,739
Investments [Member] | U.S. government and agency securities [Member]    
Assets    
Total investments 24,753 20,579
Investments [Member] | Asset backed securities [Member]    
Assets    
Total investments   312
Long-term restricted cash [Member] | Cash [Member]    
Assets    
Long-term restricted cash 4,500 4,500
Level 1 [Member]    
Assets    
Total assets measured at fair value 26,783 19,016
Level 1 [Member] | Cash and cash equivalents [Member]    
Assets    
Total cash and cash equivalents 22,283 14,516
Level 1 [Member] | Cash and cash equivalents [Member] | Cash and money market funds [Member]    
Assets    
Total cash and cash equivalents 22,283 14,516
Level 1 [Member] | Long-term restricted cash [Member] | Cash [Member]    
Assets    
Long-term restricted cash 4,500 4,500
Level 2 [Member]    
Assets    
Total assets measured at fair value 61,682 57,462
Level 2 [Member] | Cash and cash equivalents [Member]    
Assets    
Total cash and cash equivalents 3,648 2,249
Level 2 [Member] | Cash and cash equivalents [Member] | Commercial paper [Member]    
Assets    
Total cash and cash equivalents 3,648 2,249
Level 2 [Member] | Investments [Member]    
Assets    
Total investments 58,034 55,213
Level 2 [Member] | Investments [Member] | Commercial paper [Member]    
Assets    
Total investments 23,882 23,583
Level 2 [Member] | Investments [Member] | Corporate debt securities [Member]    
Assets    
Total investments 9,399 10,739
Level 2 [Member] | Investments [Member] | U.S. government and agency securities [Member]    
Assets    
Total investments 24,753 20,579
Level 2 [Member] | Investments [Member] | Asset backed securities [Member]    
Assets    
Total investments   312
Level 3 [Member]    
Liabilities    
Total liabilities measured at fair value 170 356
Level 3 [Member] | Financing Derivative [Member]    
Liabilities    
Total liabilities measured at fair value $ 170 $ 356