Income taxes Income taxes - Schedule of Components of Income Tax Expense (Benefit) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Tax Disclosure [Abstract]
Provision for income taxes (in millions)	$ 14.1	$ 26.8	$ 37.2	$ 39.6
Effective tax rate	65.90%	48.70%	341.30%	(187.70%)