Components of accumulated other comprehensive loss - Reclassifications out of AOCI (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Cost of goods sold	$ 493.6	$ 359.2	$ 1,369.9	$ 1,074.5
Selling, general and administrative expenses	8.8	14.0	31.5	34.0
Other operating expense, net	0.5	(0.4)	0.0	(1.6)
Income tax effect	1.7	3.3	3.0	4.5
Net of tax	(20.3)	20.8	(1.2)	12.8
Reclassification out of Accumulated Other Comprehensive Income [Member] | Defined benefit plan and other postretirement liabilities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Cost of goods sold	0.5	1.0	3.9	0.8
Selling, general and administrative expenses	(0.4)	0.1	(0.9)	1.1
Other operating expense, net	0.4	0.4	1.3	1.3
Income tax effect	(0.4)	(0.4)	(1.1)	(1.2)
Net of tax	0.1	1.1	3.2	2.0
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrealized gain (loss) on financial instruments [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Cost of goods sold	0.0	0.0	(0.1)	(0.1)
Income tax effect	0.0	0.0	0.0	0.0
Net of tax	$ 0.0	$ 0.0	$ (0.1)	$ (0.1)