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Related party transactions (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Dec. 31, 2019
Related party transactions          
Deferred revenue $ 1.7   $ 1.7   $ 1.7
Tufts | License Agreement          
Related party transactions          
Royalty expense 0.2 $ 0.2 0.4 $ 0.4  
Harvard University | Maximum          
Related party transactions          
Related party revenue $ 0.1 $ 0.1 $ 0.1 $ 0.1