Income taxes - Schedule of Components of Income Tax Expense (Benefit) (Details) - USD ($) $ in Millions |
3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|
Sep. 30, 2016 |
Sep. 30, 2015 |
Sep. 30, 2016 |
Sep. 30, 2015 |
|
| Income Tax Disclosure [Abstract] | ||||
| Provision for income taxes (in millions) | $ 19.5 | $ 14.1 | $ 43.4 | $ 37.2 |
| Effective tax rate | 43.00% | 65.90% | 60.10% | 341.30% |