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Summary of significant accounting policies - Deferred Contract Costs (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Summary of significant accounting policies        
Total amortization of contract costs $ 2.2 $ 1.5 $ 4.2 $ 2.8
Impairment losses $ 0.0 $ 0.0 $ 0.0 $ 0.0