Share-based compensation plans - Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 2.5	$ 2.7	$ 11.0	$ 16.2
Income tax benefit	(0.8)	(0.9)	(3.7)	(5.5)
Share-based payment expense, net of tax	1.7	1.8	7.3	10.7
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	1.6	2.7	6.1	6.2
Market Share Units MSU
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	0.2	(0.2)	2.1	1.6
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	0.6	0.6	2.5	5.6
Deferred Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	0.1	0.1	0.3	0.6
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 0.0	$ (0.5)	$ 0.0	$ 2.2