Quarterly report pursuant to Section 13 or 15(d)

Cash, Cash Equivalents and Short-Term Investments - Gross Unrealized Gains and Losses (Details)

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Cash, Cash Equivalents and Short-Term Investments - Gross Unrealized Gains and Losses (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Cash equivalents and available-for-sale securities    
Amortized Cost $ 125,912 $ 112,456
Gross Unrealized Gains 7 1
Gross Unrealized Losses (56) (83)
Fair Value 125,863 112,374
U.S. treasury bills    
Cash equivalents and available-for-sale securities    
Amortized Cost 5,247 6,733
Gross Unrealized Losses (7) (7)
Fair Value 5,240 6,726
Government-sponsored enterprises securities    
Cash equivalents and available-for-sale securities    
Amortized Cost 4,194 7,835
Gross Unrealized Losses (10) (9)
Fair Value 4,184 7,826
Corporate bonds and commercial paper    
Cash equivalents and available-for-sale securities    
Amortized Cost 116,471 97,888
Gross Unrealized Gains 7 1
Gross Unrealized Losses (39) (67)
Fair Value $ 116,439 $ 97,822