Quarterly report pursuant to Section 13 or 15(d)

Cash, Cash Equivalents and Marketable Securities (Details)

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Cash, Cash Equivalents and Marketable Securities (Details)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
USD ($)
item
Dec. 31, 2017
USD ($)
Sep. 30, 2018
USD ($)
item
Sep. 30, 2018
USD ($)
item
Cash Equivalents and Marketable Securities        
Marketable Securities, Unrealized Losses $ (13) $ (42) $ (13) $ (13)
Total, Amortized Cost 113,244 72,091 113,244 113,244
Total, Fair Value $ 113,231 $ 72,049 113,231 113,231
Realized gains (losses)     $ 0 $ 0
Number of investments adjusted for other-than-temporary declines in fair value | item 0   0 0
Unrealized Loss Position        
Number of securities in unrealized loss position for less than twelve months | item 14   14 14
Fair value of securities held by Company in an unrealized loss position for less than twelve months $ 69,700   $ 69,700 $ 69,700
Number of securities in unrealized loss position for more than twelve months | item 0   0 0
Number of securities with an other-than-temporary impairment | item 0   0 0
Maximum        
Cash Equivalents and Marketable Securities        
Maturity period of marketable securities 12 months 12 months    
U.S treasury obligations        
Cash Equivalents and Marketable Securities        
Marketable Securities, Amortized Cost $ 69,720 $ 39,886 $ 69,720 $ 69,720
Marketable Securities, Unrealized Losses (13) (42) (13) (13)
Marketable Securities, Fair Value 69,707 39,844 69,707 69,707
Cash and money market funds        
Cash Equivalents and Marketable Securities        
Cash Equivalents, Amortized Cost 20,524 17,205 20,524 20,524
Cash Equivalents, Fair value 20,524 17,205 20,524 20,524
Overnight repurchase agreements        
Cash Equivalents and Marketable Securities        
Cash Equivalents, Amortized Cost 23,000 15,000 23,000 23,000
Cash Equivalents, Fair value $ 23,000 $ 15,000 $ 23,000 $ 23,000