Income taxes - Reconciliation of Changes in Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at January 1	$ 30.9	$ 25.6	$ 18.9
Additions based on tax positions related to the current year	3.5	8.5	11.1
Changes for tax positions of prior years	(8.8)	(0.7)	(0.6)
Reductions as a result of a lapse of applicable statute of limitations	(2.6)	(2.3)	(2.3)
Settlements	(0.3)	(1.8)	(1.1)
Changes due to foreign currency	(1.1)	1.6	(0.4)
Balance at December 31	$ 21.6	$ 30.9	$ 25.6