Income taxes - Valuation Allowance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Movement In Deferred Tax Assets, Valuation Allowance [Roll Forward]
Valuation allowance, at beginning of year	$ (1,110.5)	$ (1,226.2)	$ (1,271.5)
Increase (decrease) in valuation allowance	(2.1)
Valuation allowance, at end of year	(1,150.1)	(1,110.5)	(1,226.2)
Change in valuation allowances	2.1	(9.8)	(0.8)
Actuarial pension adjustments
Movement In Deferred Tax Assets, Valuation Allowance [Roll Forward]
Increase (decrease) in valuation allowance	84.5	70.7	99.5
Expired net operating losses/tax credits
Movement In Deferred Tax Assets, Valuation Allowance [Roll Forward]
Increase (decrease) in valuation allowance	42.9	52.3	50.0
Foreign exchange
Movement In Deferred Tax Assets, Valuation Allowance [Roll Forward]
Increase (decrease) in valuation allowance	(6.8)	14.8	18.4
Recognition of net income tax benefit
Movement In Deferred Tax Assets, Valuation Allowance [Roll Forward]
Increase (decrease) in valuation allowance	(125.9)	(43.9)	(102.1)
Other
Movement In Deferred Tax Assets, Valuation Allowance [Roll Forward]
Increase (decrease) in valuation allowance	(34.3)	21.8	(20.5)
US Pension Activity
Movement In Deferred Tax Assets, Valuation Allowance [Roll Forward]
Change in valuation allowances	$ (95.9)	$ (11.3)	$ (84.9)