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Investments (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Available-for-sale securities    
Amortized Cost $ 85,284 $ 112,402
Gross Unrealized Gains 0 5
Gross Unrealized Losses (167) (198)
Fair Value 85,117 112,209
Short-term investments    
Available-for-sale securities    
Amortized Cost 82,280 90,050
Gross Unrealized Gains 1
Gross Unrealized Losses (154) (134)
Fair Value 82,126 89,917
Accrued interest 600 300
Long-term investments    
Available-for-sale securities    
Amortized Cost 3,004 22,352
Gross Unrealized Gains 4
Gross Unrealized Losses (13) (64)
Fair Value 2,991 22,292
Accrued interest $ 14 100
Long-term investments | Minimum    
Available-for-sale securities    
Maturity range of long term investment 12 months  
Long-term investments | Maximum    
Available-for-sale securities    
Maturity range of long term investment 13 months  
Corporate debt    
Available-for-sale securities    
Amortized Cost $ 23,507 51,354
Gross Unrealized Losses (36) (121)
Fair Value 23,471 51,233
U.S. Treasury notes    
Available-for-sale securities    
Amortized Cost 61,777 61,048
Gross Unrealized Gains 0 5
Gross Unrealized Losses (131) (77)
Fair Value $ 61,646 $ 60,976