Annual report pursuant to Section 13 and 15(d)

Cash Equivalents and Marketable Securities (Details)

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Cash Equivalents and Marketable Securities (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
USD ($)
item
Dec. 31, 2016
USD ($)
Cash Equivalents and Marketable Securities    
Marketable Securities, Unrealized Losses $ (42) $ (9)
Total, Amortized Cost 72,091 83,602
Total, Fair Value 72,049 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 17  
Fair value of securities held by Company in an unrealized loss position for less than twelve months $ 39,800  
Number of securities in unrealized loss position for more than twelve months | item 0  
Number of securities with an other-than-temporary impairment | item 0  
Money market funds    
Cash Equivalents and Marketable Securities    
Cash Equivalents, Amortized Cost $ 17,205 58,588
Cash Equivalents, Fair value 17,205 58,588
Overnight repurchase agreements    
Cash Equivalents and Marketable Securities    
Cash Equivalents, Amortized Cost 15  
Cash Equivalents, Fair value 15  
U.S treasury obligations    
Cash Equivalents and Marketable Securities    
Marketable Securities, Amortized Cost 39,886 25,014
Marketable Securities, Unrealized Losses (42) (9)
Marketable Securities, Fair Value $ 39,844 $ 25,005