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%