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, 2019
USD ($)
item
Dec. 31, 2018
USD ($)
item
Dec. 31, 2017
USD ($)
item
Cash Equivalents and Marketable Securities      
Marketable Securities, Amortized Cost $ 91,392,000 $ 99,682,000  
Marketable Securities, Unrealized Gains 24,000    
Marketable Securities, Unrealized Losses   (3,000)  
Marketable Securities, Fair Value 91,416,000 99,679,000  
Realized gains (losses) $ 0 $ 0 $ 0
Number of investments adjusted for other-than-temporary declines in fair value | item 0 0 0
Number of securities in unrealized loss position for less than twelve months | item 0    
U.S treasury obligations      
Cash Equivalents and Marketable Securities      
Marketable Securities, Amortized Cost $ 49,951,000 $ 49,796,000  
Marketable Securities, Unrealized Gains 24,000    
Marketable Securities, Unrealized Losses   (3,000)  
Marketable Securities, Fair Value 49,975,000 49,793,000  
Cash and money market funds      
Cash Equivalents and Marketable Securities      
Cash Equivalents, Amortized Cost 29,441,000 34,886,000  
Cash Equivalents, Fair value 29,441,000 34,886,000  
Overnight repurchase agreements      
Cash Equivalents and Marketable Securities      
Cash Equivalents, Amortized Cost 12,000,000 15,000,000  
Cash Equivalents, Fair value $ 12,000,000 $ 15,000,000