Income taxes - Valuation Allowance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Movement In Deferred Tax Assets, Valuation Allowance [Roll Forward]
Valuation allowance, at beginning of year	$ (1,226.2)	$ (1,271.5)	$ (1,524.7)
Valuation allowance, at end of year	(1,110.5)	(1,226.2)	(1,271.5)
Change in valuation allowances	(9.8)	(0.8)	2.9
Actuarial pension adjustments
Movement In Deferred Tax Assets, Valuation Allowance [Roll Forward]
Increase (decrease) in valuation allowance	70.7	99.5	41.8
Expired net operating losses/tax credits
Movement In Deferred Tax Assets, Valuation Allowance [Roll Forward]
Increase (decrease) in valuation allowance	52.3	50.0	28.9
Foreign exchange
Movement In Deferred Tax Assets, Valuation Allowance [Roll Forward]
Increase (decrease) in valuation allowance	14.8	18.4	(20.9)
Recognition of net income tax benefit
Movement In Deferred Tax Assets, Valuation Allowance [Roll Forward]
Increase (decrease) in valuation allowance	(43.9)	(102.1)	189.0
Other
Movement In Deferred Tax Assets, Valuation Allowance [Roll Forward]
Increase (decrease) in valuation allowance	21.8	(20.5)	14.4
US Pension Activity
Movement In Deferred Tax Assets, Valuation Allowance [Roll Forward]
Change in valuation allowances	$ (11.3)	$ (84.9)	$ 141.7