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)
 12 Months Ended
Dec. 31, 2018
USD ($)
item
Dec. 31, 2017
USD ($)
Cash Equivalents and Marketable Securities    
Marketable Securities, Amortized Cost $ 99,682,000 $ 72,091,000
Marketable Securities, Unrealized Losses (3,000) (42,000)
Marketable Securities, Fair Value 99,679,000 72,049,000
Realized gains (losses) $ 0  
Number of investments adjusted for other-than-temporary declines in fair value | item 0  
Fair value of securities held by Company in an unrealized loss position for less than twelve months $ 34,800,000  
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  
Number of securities in unrealized loss position for less than twelve months | item 7  
U.S treasury obligations    
Cash Equivalents and Marketable Securities    
Marketable Securities, Amortized Cost $ 49,796,000 39,886,000
Marketable Securities, Unrealized Losses (3,000) (42,000)
Marketable Securities, Fair Value 49,793,000 39,844,000
Cash and money market funds    
Cash Equivalents and Marketable Securities    
Cash Equivalents, Amortized Cost 34,886,000 17,205,000
Cash Equivalents, Fair value 34,886,000 17,205,000
Overnight repurchase agreements    
Cash Equivalents and Marketable Securities    
Cash Equivalents, Amortized Cost 15,000,000 15,000,000
Cash Equivalents, Fair value $ 15,000,000 $ 15,000,000