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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Operating income (loss)									$ 284.1	$ 97.1	[1]	$ 129.2	[1]
Interest expense									(64.0)	(52.8)		(27.4)
Other income (expense), net									(76.9)	(116.4)	[1]	(81.3)	[1]
Cost reduction charges	$ (22.8)	$ 0.9	$ (0.7)	$ 2.9	$ (47.8)	$ (46.1)	$ (27.5)	$ (25.4)
Income (loss) before income taxes	$ 38.1	$ 22.2	$ 20.3	$ 62.6	$ 27.4	$ (40.4)	$ (42.3)	$ (16.8)	143.2	(72.1)		20.5
Operating Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Operating income (loss)									305.4	235.4		208.4
Segment Reconciling Items
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Cost reduction charges									(19.7)	(135.0)		(82.1)
Corporate and eliminations
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Operating income (loss)									$ (1.6)	(3.3)		$ 2.9
Other income (expense), net | Net foreign currency translation adjustment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Cost reduction charges										$ 11.8

[1] Amounts were changed to conform to the current-year presentation. See Note 2.