Segment information - Reconciliation of Segment Operating Income to Consolidated Income Before Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Operating profit (loss)									$ 47.6	$ (55.1)	$ 154.9
Interest expense									(27.4)	(11.9)	(9.2)
Other income (expense), net									0.3	8.2	(0.2)
Cost reduction charges	$ (13.1)	$ (31.9)	$ (10.2)	$ (26.9)	$ (48.5)	$ (17.4)	$ (52.6)		(82.1)	(118.5)
Income (loss) before income taxes	$ 24.6	$ (15.2)	$ 44.3	$ (33.2)	$ 12.4	$ 7.3	$ (50.8)	$ (27.7)	20.5	(58.8)	145.5
Operating Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Operating profit (loss)									208.4	174.9	233.6
Segment Reconciling Items
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Cost reduction charges									(82.1)	(118.5)	0.0
Corporate and eliminations
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Operating profit (loss)									$ (78.7)	$ (111.5)	$ (78.7)