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Investments in marketable securities - Marketable Securities (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2023
Jun. 30, 2023
Jun. 30, 2022
Dec. 31, 2022
Debt Securities, Available-for-Sale [Abstract]        
Accrued investment income receivable $ 600 $ 600   $ 600
Realized gains or losses 100 100 $ 0  
Cash and Cash Equivalents        
Cash and Cash Equivalents [Abstract]        
Cash Equivalents, at Carrying Value 21,816 21,816   23,218
Cash Equivalents, Fair Value Disclosure 21,816 21,816   23,218
Debt Securities, Available-for-Sale [Abstract]        
Gross Unrealized Gain 0 0    
Gross Unrealized Loss 0 0    
Short Term Marketable Securities        
Debt Securities, Available-for-Sale [Abstract]        
Amortized Cost 126,468 126,468   117,674
Gross Unrealized Gain 0 0   0
Gross Unrealized Loss (1,181) (1,181)   (1,537)
Fair Value 125,287 125,287   116,137
Long Term Marketable Securities        
Debt Securities, Available-for-Sale [Abstract]        
Amortized Cost 11,160 11,160   38,129
Gross Unrealized Gain 0 0   0
Gross Unrealized Loss (103) (103)   (766)
Fair Value 11,057 11,057   37,363
Money market | Cash and Cash Equivalents        
Cash and Cash Equivalents [Abstract]        
Money Market Funds, at Carrying Value 21,816 21,816   23,218
US treasury bills | Short Term Marketable Securities        
Debt Securities, Available-for-Sale [Abstract]        
Amortized Cost 7,834 7,834   8,483
Gross Unrealized Gain 0 0   0
Gross Unrealized Loss (4) (4)   (16)
Fair Value 7,830 7,830   8,467
US treasury bills | Long Term Marketable Securities        
Debt Securities, Available-for-Sale [Abstract]        
Amortized Cost       25,433
Gross Unrealized Gain       0
Gross Unrealized Loss       (336)
Fair Value       25,097
US government agency bonds | Short Term Marketable Securities        
Debt Securities, Available-for-Sale [Abstract]        
Amortized Cost 35,624 35,624   26,686
Gross Unrealized Gain 0 0   0
Gross Unrealized Loss (314) (314)   (424)
Fair Value 35,310 35,310   26,262
US government agency bonds | Long Term Marketable Securities        
Debt Securities, Available-for-Sale [Abstract]        
Amortized Cost       3,724
Gross Unrealized Gain       0
Gross Unrealized Loss       (130)
Fair Value       3,594
US corporate bonds | Short Term Marketable Securities        
Debt Securities, Available-for-Sale [Abstract]        
Amortized Cost 56,066 56,066   27,144
Gross Unrealized Gain 0 0   0
Gross Unrealized Loss (507) (507)   (303)
Fair Value 55,559 55,559   26,841
US corporate bonds | Long Term Marketable Securities        
Debt Securities, Available-for-Sale [Abstract]        
Amortized Cost 9,161 9,161    
Gross Unrealized Gain 0 0    
Gross Unrealized Loss (66) (66)    
Fair Value 9,095 9,095    
Yankee bonds | Long Term Marketable Securities        
Debt Securities, Available-for-Sale [Abstract]        
Amortized Cost 1,999 1,999    
Gross Unrealized Gain 0 0    
Gross Unrealized Loss (37) (37)    
Fair Value 1,962 1,962    
US government bonds | Short Term Marketable Securities        
Debt Securities, Available-for-Sale [Abstract]        
Amortized Cost 26,944 26,944   55,361
Gross Unrealized Gain 0 0   0
Gross Unrealized Loss (356) (356)   (794)
Fair Value $ 26,588 $ 26,588   54,567
US government bonds | Long Term Marketable Securities        
Debt Securities, Available-for-Sale [Abstract]        
Amortized Cost       8,972
Gross Unrealized Gain       0
Gross Unrealized Loss       (300)
Fair Value       $ 8,672