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Cash, Cash Equivalents And Investments (Summary Of Cash, Cash Equivalents And Investments) (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost $ 102,612 $ 71,973
Gross unrealized gains 2 13
Gross unrealized losses (18) (8)
Fair value 102,596 71,978
Cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 36,479 16,765
Gross unrealized gains
Gross unrealized losses (1)
Fair value 36,478 16,765
Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 66,133 55,208
Gross unrealized gains 2 13
Gross unrealized losses (17) (8)
Fair value 66,118 55,213
Cash and money market funds [Member] | Cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 17,160 14,516
Gross unrealized gains
Gross unrealized losses
Fair value 17,160 14,516
Commercial paper [Member] | Cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 19,319 2,249
Gross unrealized gains
Gross unrealized losses (1)
Fair value 19,318 2,249
Commercial paper [Member] | Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 29,640 23,581
Gross unrealized gains 2 5
Gross unrealized losses (2) (3)
Fair value 29,640 23,583
Corporate debt securities [Member] | Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 9,718 10,741
Gross unrealized gains 1
Gross unrealized losses (3) (3)
Fair value 9,715 10,739
Asset backed securities [Member] | Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost   312
Gross unrealized gains
Gross unrealized losses
Fair value   312
U.S. government and agency securities [Member] | Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 26,775 20,574
Gross unrealized gains 7
Gross unrealized losses (12) (2)
Fair value 26,763 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