Income taxes - Changes in Amount of Uncertain Tax Positions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income taxes
Unrecognized tax benefits at beginning of year	$ 3.2	$ 3.8	$ 4.1
Net increase (decrease):
Tax positions taken in current period	0.5	0.7	0.6
Lapse due to applicable statute of limitations	(1.0)	(1.1)	(0.7)
Change in currency exchange rates	0.1	(0.2)	(0.2)
Unrecognized tax benefits at end of year	$ 2.8	$ 3.2	$ 3.8