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Employee benefit plans - Defined Contribution Plan (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Employee benefit plans        
Defined contribution expense $ 3.0 $ 2.6 $ 9.0 $ 8.1