Income taxes - Reconciliation of Changes in Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at January 1	$ 17.8	$ 21.6	$ 30.9
Additions based on tax positions related to the current year	4.6	1.9	3.5
Changes for tax positions of prior years	2.6	1.2	(8.8)
Reductions as a result of a lapse of applicable statute of limitations	(4.6)	(5.4)	(2.6)
Settlements	0.0	0.0	(0.3)
Changes due to foreign currency	0.5
Changes due to foreign currency		(1.5)	(1.1)
Balance at December 31	$ 20.9	$ 17.8	$ 21.6