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Revenue recognition - Narrative (Details)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2019
segment
Dec. 31, 2019
USD ($)
segment
Dec. 31, 2018
USD ($)
Concentration Risk [Line Items]      
Number of operating segments | segment 1 1  
Revenue, remaining performance obligation, amount   $ 600.0  
Deferred costs, current   $ 34.0 $ 1.2
Customer Concentration Risk | Accounts Receivable      
Concentration Risk [Line Items]      
Concentration risk percentage   69.00% 76.00%
Customer Concentration Risk | Accounts Receivable | USG      
Concentration Risk [Line Items]      
Concentration risk percentage   69.00% 76.00%