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Summary of significant accounting policies - Revenue Recognition (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Disaggregation of Revenue [Line Items]          
Deferred revenue $ 3.9   $ 3.9   $ 4.4
Revenue recognized included in deferred revenue in prior period 0.7 $ 0.2 3.6 $ 0.8  
Prepaid expenses and other current asset | Barda Agreement          
Disaggregation of Revenue [Line Items]          
Contract assets $ 1.1   $ 1.1   $ 0.5