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Revenue from contracts with customers - Revenue streams - Parenthetical (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Intersegment Transaction        
Revenue From Contract With Customer Excluding Assessed Tax $ 1.9 $ 1.3 $ 2.1 $ 1.7