Income taxes - Reconciliation of Changes in Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at January 1	$ 25.6	$ 18.9	$ 27.9
Additions based on tax positions related to the current year	8.5	11.1	2.6
Changes for tax positions of prior years	(0.7)	(0.6)	(6.1)
Reductions as a result of a lapse of applicable statute of limitations	(2.3)	(2.3)	(2.4)
Settlements	(1.8)	(1.1)	(1.5)
Changes due to foreign currency	1.6	(0.4)	(1.6)
Balance at December 31	$ 30.9	$ 25.6	$ 18.9