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Marketable Securities - Summary of Investments (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Marketable securities available-for-sale    
Amortization Cost $ 71,275 $ 60,584
Gross Unrealized Gains 58  
Gross Unrealized Losses   (8)
Estimated Fair Value 71,333 60,576
U.S. Treasury obligations    
Marketable securities available-for-sale    
Amortization Cost 71,275 60,584
Gross Unrealized Gains 58  
Gross Unrealized Losses   (8)
Estimated Fair Value $ 71,333 $ 60,576