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Cash, Cash Equivalents And Investments (Summary Of Cash, Cash Equivalents And Investments) (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost $ 56,053 $ 71,973
Gross unrealized gains 3 13
Gross unrealized losses (6) (8)
Fair value 56,050 71,978
Cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 28,982 16,765
Gross unrealized gains
Gross unrealized losses
Fair value 28,982 16,765
Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 27,071 55,208
Gross unrealized gains 3 13
Gross unrealized losses (6) (8)
Fair value 27,068 55,213
Cash and money market funds [Member] | Cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 27,183 14,516
Gross unrealized gains
Gross unrealized losses
Fair value 27,183 14,516
Commercial paper [Member] | Cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 1,799 2,249
Gross unrealized gains
Gross unrealized losses
Fair value 1,799 2,249
Commercial paper [Member] | Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 18,691 23,581
Gross unrealized gains 2 5
Gross unrealized losses (2) (3)
Fair value 18,691 23,583
Corporate debt securities [Member] | Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 6,347 10,741
Gross unrealized gains 1 1
Gross unrealized losses (2) (3)
Fair value 6,346 10,739
Asset backed securities [Member] | Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 27 312
Gross unrealized gains
Gross unrealized losses
Fair value 27 312
U.S. government and agency securities [Member] | Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 2,006 20,574
Gross unrealized gains 7
Gross unrealized losses (2) (2)
Fair value 2,004 20,579
Cash [Member] | Long-term restricted cash [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 4,500 4,500
Gross unrealized gains
Gross unrealized losses
Fair value $ 4,500 $ 4,500