Income taxes - Schedule of Components of Income Tax Expense (Benefit) (Details) - USD ($) $ in Millions |
1 Months Ended | 3 Months Ended | 9 Months Ended | |||
|---|---|---|---|---|---|---|
Oct. 31, 2018 |
Sep. 30, 2018 |
Dec. 31, 2017 |
Sep. 30, 2017 |
Sep. 30, 2018 |
Sep. 30, 2017 |
|
| Subsequent Event [Line Items] | ||||||
| Provision for income taxes (in millions) | $ 23.0 | $ 16.4 | $ 53.0 | $ 48.1 | ||
| Effective tax rate | 54.90% | 43.70% | (514.60%) | 39.00% | ||
| Tax Reform Act Tax Expense Charge | $ 92.0 | |||||
| Foreign tax credit amount | $ 31.1 | |||||
| Subsequent Event | ||||||
| Subsequent Event [Line Items] | ||||||
| Foreign tax credit amount | $ 1.3 | |||||