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Assets - Available-For-Sale Securities (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Total available-for-sale securities    
Amortized Cost $ 65,164 $ 47,807
Gross Unrealized Gains 1  
Gross Unrealized Losses (35) (27)
Estimated Fair Value 65,130 47,780
Available-for-sale securities in a loss position    
Estimated Fair Value 47,797 44,781
Investments that had been in a continuous loss position for more than 12 months 0 0
Realized gains 0 0
U.S. treasuries    
Total available-for-sale securities    
Amortized Cost 42,245 33,914
Gross Unrealized Losses (17) (19)
Estimated Fair Value 42,228 33,895
Available-for-sale securities in a loss position    
Estimated Fair Value 37,275 33,895
Corporate debt    
Total available-for-sale securities    
Amortized Cost 10,540 10,894
Gross Unrealized Losses (18) (8)
Estimated Fair Value 10,522 10,886
Available-for-sale securities in a loss position    
Estimated Fair Value 10,522 10,886
Commercial paper    
Total available-for-sale securities    
Amortized Cost 12,379 2,999
Gross Unrealized Gains 1  
Estimated Fair Value $ 12,380 $ 2,999