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 6 Months Ended
Jun. 30, 2018
USD ($)
item
Jun. 30, 2017
USD ($)
item
Jun. 30, 2018
USD ($)
item
Jun. 30, 2017
USD ($)
item
Dec. 31, 2017
USD ($)
Cash Equivalents and Marketable Securities          
Marketable Securities, Unrealized Losses $ (7)   $ (7)   $ (42)
Total, Amortized Cost 124,373   124,373   72,091
Total, Fair Value 124,366   124,366   72,049
Realized gains (losses) $ 0 $ 0 $ 0 $ 0  
Number of investments adjusted for other-than-temporary declines in fair value | item 0 0 0 0  
Unrealized Loss Position          
Number of securities in unrealized loss position for less than twelve months | item 4   4    
Fair value of securities held by Company in an unrealized loss position for less than twelve months $ 10,000   $ 10,000    
Number of securities in unrealized loss position for more than twelve months | item 0   0    
U.S treasury obligations          
Cash Equivalents and Marketable Securities          
Marketable Securities, Amortized Cost $ 9,987   $ 9,987   39,886
Marketable Securities, Unrealized Losses (7)   (7)   (42)
Marketable Securities, Fair Value 9,980   9,980   39,844
Money market funds          
Cash Equivalents and Marketable Securities          
Cash Equivalents, Amortized Cost 46,386   46,386   17,205
Cash Equivalents, Fair value 46,386   46,386   17,205
Overnight repurchase agreements          
Cash Equivalents and Marketable Securities          
Cash Equivalents, Amortized Cost 68,000   68,000   15,000
Cash Equivalents, Fair value $ 68,000   $ 68,000   $ 15,000