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Income taxes - Current income tax provision (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Current income tax provision:        
Income tax benefit related to operating losses and research and development tax credits $ 0 $ 0 $ 0 $ 0
Income tax benefits $ 0   $ 0