Stockholders' equity - Amounts Reclassified Out of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other (expense), net	$ 393.9	$ 82.4	$ 580.3
Loss before income taxes	347.8	62.6	461.7
Income tax	79.3	42.3	(11.9)
Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Loss before income taxes	420.3	156.1	676.1
Income tax	(1.8)	(6.0)	(59.3)
Consolidated net loss	418.5	150.1	616.8
Reclassification out of Accumulated Other Comprehensive Income | Translation Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other (expense), net	(3.7)	2.9	4.0
Reclassification out of Accumulated Other Comprehensive Income | Amortization of prior service credit
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other (expense), net	(5.4)	(5.8)	(6.2)
Reclassification out of Accumulated Other Comprehensive Income | Amortization of actuarial losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other (expense), net	80.5	159.0	178.9
Reclassification out of Accumulated Other Comprehensive Income | Settlement losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other (expense), net	$ 348.9	$ 0.0	$ 499.4