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
Mar. 31, 2018
Dec. 31, 2017
Cash equivalents and available-for-sale securities    
Amortized Cost $ 86,715 $ 112,456
Gross Unrealized Gains 1 1
Gross Unrealized Losses (88) (83)
Fair Value 86,628 112,374
U.S. treasury bills    
Cash equivalents and available-for-sale securities    
Amortized Cost 6,741 6,733
Gross Unrealized Losses (12) (7)
Fair Value 6,729 6,726
Government-sponsored enterprises securities    
Cash equivalents and available-for-sale securities    
Amortized Cost 6,192 7,835
Gross Unrealized Losses (13) (9)
Fair Value 6,179 7,826
Corporate bonds and commercial paper    
Cash equivalents and available-for-sale securities    
Amortized Cost 73,782 97,888
Gross Unrealized Gains 1 1
Gross Unrealized Losses (63) (67)
Fair Value $ 73,720 $ 97,822