Quarterly report pursuant to Section 13 or 15(d)

Cash Equivalents and Marketable Securities (Details)

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Cash Equivalents and Marketable Securities (Details)
$ in Thousands
 9 Months Ended
Sep. 30, 2017
USD ($)
item
Dec. 31, 2016
USD ($)
Cash Equivalents and Marketable Securities    
Marketable Securities, Unrealized Losses $ (10) $ (9)
Total, Amortized Cost 81,958 83,602
Total, Fair Value 81,948 83,593
Realized gains (losses) $ 0  
Number of investments adjusted for other-than-temporary declines in fair value | item 0  
Unrealized Loss Position    
Number of securities in unrealized loss position for less than twelve months | item 13  
Fair value of securities held by Company in an unrealized loss position for less than twelve months $ 32,300  
Number of securities in unrealized loss position for more than twelve months | item 0  
Money market funds    
Cash Equivalents and Marketable Securities    
Cash Equivalents, Amortized Cost $ 12,149 58,588
Cash Equivalents, Fair value 12,149 58,588
Overnight repurchase agreements    
Cash Equivalents and Marketable Securities    
Cash Equivalents, Amortized Cost 30,000  
Cash Equivalents, Fair value 30,000  
U.S treasury obligations    
Cash Equivalents and Marketable Securities    
Marketable Securities, Amortized Cost 39,809 25,014
Marketable Securities, Unrealized Losses (10) (9)
Marketable Securities, Fair Value $ 39,799 $ 25,005