Accumulated other comprehensive income (loss) - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of revenues	$ 604.5	$ 620.9	$ 1,201.5	$ 1,250.0
Selling, general and administrative expenses	106.7	120.0	217.0	232.3
Other operating expense	6.2	34.0	6.9	55.8
Interest and other income (expense)	0.7	0.4	0.7	0.8
Reclassification out of accumulated other comprehensive income | Benefit Plan Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of revenues	10.5	8.8	20.9	18.7
Selling, general and administrative expenses	2.3	3.7	4.4	7.4
Reclassification out of accumulated other comprehensive income | Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other operating expense	0.8	0.4	1.4	(1.9)
Interest and other income (expense)	$ (0.1)	$ (0.1)	$ (0.2)	$ (0.3)