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Investments (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2017
Dec. 31, 2016
Available-for-sale securities    
Amortized Cost $ 111,494 $ 112,402
Gross Unrealized Gains 1 5
Gross Unrealized Losses (184) (198)
Estimated Fair Value 111,311 112,209
Short-term investments    
Available-for-sale securities    
Amortized Cost 94,891 90,050
Gross Unrealized Gains 1 1
Gross Unrealized Losses (167) (134)
Estimated Fair Value 94,725 89,917
Accrued interest 400 300
Long-term investments    
Available-for-sale securities    
Amortized Cost 16,603 22,352
Gross Unrealized Gains 0 4
Gross Unrealized Losses (17) (64)
Estimated Fair Value 16,586 22,292
Accrued interest $ 100 100
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 15 months  
Corporate debt    
Available-for-sale securities    
Amortized Cost $ 43,185 51,354
Gross Unrealized Gains 1 0
Gross Unrealized Losses (66) (121)
Estimated Fair Value 43,120 51,233
U.S. Treasury notes    
Available-for-sale securities    
Amortized Cost 68,309 61,048
Gross Unrealized Gains 0 5
Gross Unrealized Losses (118) (77)
Estimated Fair Value $ 68,191 $ 60,976