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
Sep. 30, 2018
Dec. 31, 2017
Cash equivalents and available-for-sale securities    
Amortized Cost $ 103,096 $ 112,456
Gross Unrealized Gains 1 1
Gross Unrealized Losses (26) (83)
Fair Value 103,071 112,374
U.S. treasury bills    
Cash equivalents and available-for-sale securities    
Amortized Cost 4,250 6,733
Gross Unrealized Losses (2) (7)
Fair Value 4,248 6,726
Government-sponsored enterprises securities    
Cash equivalents and available-for-sale securities    
Amortized Cost 14,493 7,835
Gross Unrealized Losses (4) (9)
Fair Value 14,489 7,826
Corporate bonds and commercial paper    
Cash equivalents and available-for-sale securities    
Amortized Cost 84,353 97,888
Gross Unrealized Gains 1 1
Gross Unrealized Losses (20) (67)
Fair Value $ 84,334 $ 97,822