Quarterly report pursuant to Section 13 or 15(d)

Cash, Cash Equivalents and Short-Term Investments - Unrealized Loss Position (Details)

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Cash, Cash Equivalents and Short-Term Investments - Unrealized Loss Position (Details)
$ in Thousands
 6 Months Ended
Jun. 30, 2018
USD ($)
item
Jun. 30, 2018
USD ($)
item
Fair value and gross unrealized losses of investments in individual securities in unrealized loss position    
Weighted-average time to maturity of cash equivalents and available-for-sale securities 85 days  
Number of investments in continuous unrealized loss position for more than twelve months | item 0 0
Number of individual securities in unrealized loss position for twelve months or less | item 27 27
Other-than-temporary impairments of securities $ 0  
Fair Value 43,078 $ 43,078
Unrealized Losses   (56)
U.S. treasury bills    
Fair value and gross unrealized losses of investments in individual securities in unrealized loss position    
Fair Value 5,240 5,240
Unrealized Losses   (7)
Government-sponsored enterprises securities    
Fair value and gross unrealized losses of investments in individual securities in unrealized loss position    
Fair Value 4,184 4,184
Unrealized Losses   (10)
Corporate bonds and commercial paper    
Fair value and gross unrealized losses of investments in individual securities in unrealized loss position    
Fair Value $ 33,654 33,654
Unrealized Losses   $ (39)