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
 3 Months Ended
Mar. 31, 2018
USD ($)
item
Dec. 31, 2017
USD ($)
Cash Equivalents and Marketable Securities    
Marketable Securities, Unrealized Losses $ (34) $ (42)
Total, Amortized Cost 121,775 72,091
Total, Fair Value 121,741 72,049
Realized gains (losses) $ 0  
Unrealized Loss Position    
Number of securities in unrealized loss position for less than twelve months | item 11  
Fair value of securities held by Company in an unrealized loss position for less than twelve months $ 27,400  
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 $ 36,334 17,205
Cash Equivalents, Fair value 36,334 17,205
Overnight repurchase agreements    
Cash Equivalents and Marketable Securities    
Cash Equivalents, Amortized Cost 58,000 15,000
Cash Equivalents, Fair value 58,000 15,000
U.S treasury obligations    
Cash Equivalents and Marketable Securities    
Marketable Securities, Amortized Cost 27,441 39,886
Marketable Securities, Unrealized Losses (34) (42)
Marketable Securities, Fair Value $ 27,407 $ 39,844