Income taxes - Reconciliation of unrecognized tax benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Income Tax Disclosure
Beginning Balance	$ 3.5	$ 14.8
Reduction as a result of lapse of statute of limitations	(1.0)	(11.3)
Ending Balance	$ 2.5	$ 3.5