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Investments (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Available-for-sale securities    
Amortized Cost $ 216,098 $ 85,284
Gross Unrealized Gains 0 0
Gross Unrealized Losses (425) (167)
Fair Value 215,673 85,117
Short-term investments    
Available-for-sale securities    
Amortized Cost 162,662 82,280
Gross Unrealized Gains 0 0
Gross Unrealized Losses (282) (154)
Fair Value 162,380 82,126
Accrued interest 700 600
Long-term investments    
Available-for-sale securities    
Amortized Cost 53,436 3,004
Gross Unrealized Gains 0 0
Gross Unrealized Losses (143) (13)
Fair Value 53,293 2,991
Accrued interest $ 200 14
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 18 months  
Corporate debt    
Available-for-sale securities    
Amortized Cost $ 28,221 23,507
Gross Unrealized Gains 0 0
Gross Unrealized Losses (75) (36)
Fair Value 28,146 23,471
U.S. Treasury notes    
Available-for-sale securities    
Amortized Cost 187,877 61,777
Gross Unrealized Gains 0 0
Gross Unrealized Losses (350) (131)
Fair Value 187,527 $ 61,646
Commercial paper    
Available-for-sale securities    
Amortized Cost 0  
Gross Unrealized Gains 0  
Gross Unrealized Losses 0  
Fair Value $ 0