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Cash And Cash Equivalents And Investments (Summary Of Cash, Cash Equivalents And Investments) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost $ 62,904 $ 71,973
Gross unrealized gains   13
Gross unrealized losses (32) (8)
Fair value 62,872 71,978
Cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 16,507 16,765
Fair value 16,507 16,765
Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 46,397 55,208
Gross unrealized gains   13
Gross unrealized losses (32) (8)
Fair value 46,365 55,213
Cash and money market funds [Member] | Cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 14,858 14,516
Fair value 14,858 14,516
Commercial paper [Member] | Cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 1,649 2,249
Fair value 1,649 2,249
Commercial paper [Member] | Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 20,408 23,581
Gross unrealized gains   5
Gross unrealized losses (14) (3)
Fair value 20,394 23,583
Corporate debt securities [Member] | Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 9,043 10,741
Gross unrealized gains   1
Gross unrealized losses (9) (3)
Fair value 9,034 10,739
Asset backed securities [Member] | Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost   312
Fair value   312
U.S. government and agency securities [Member] | Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 16,946 20,574
Gross unrealized gains   7
Gross unrealized losses (9) (2)
Fair value 16,937 20,579
Cash [Member] | Long-term restricted cash [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 4,500 4,500
Fair value $ 4,500 $ 4,500