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, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Operating income (loss)									$ 87.0	$ 137.9	$ 212.1
Interest expense									(29.2)	(62.1)	(64.0)
Other (expense), net									(329.6)	(136.4)	(77.1)
Cost reduction charges	$ (46.9)	$ (13.2)	$ (7.9)	$ (27.5)	$ (23.3)	$ (0.2)	$ (2.6)	$ (2.6)	(95.5)	(28.7)	(19.7)
Income (loss) from continuing operations before income taxes	$ (155.9)	$ (7.2)	$ (66.8)	$ (41.9)	$ (32.8)	$ (15.0)	$ 7.9	$ (20.7)	(271.8)	(60.6)	71.0
Operating Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Operating income (loss)									154.8	180.1	233.4
Segment Reconciling Items
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense									(29.2)	(62.1)	(64.0)
Other (expense), net									(329.6)	(136.4)	(77.1)
Cost reduction charges									(63.2)	(28.7)	(19.7)
Corporate and eliminations
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Operating income (loss)									(4.6)	(13.5)	(1.6)
Other (expenses), net
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Cost reduction charges									(32.3)	$ 0.0	$ 0.0
Other (expenses), net | Net foreign currency translation losses
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Cost reduction charges									$ 32.3