Income taxes - Reconciliation of unrecognized tax benefits (Details) - USD ($) $ in Millions	3 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022
Income Tax Disclosure
Beginning Balance	$ 2.5	$ 2.5	$ 2.5	$ 3.5	$ 3.5	$ 3.5
Reduction as a result of lapse of statute of limitations	0.0	0.0	0.0	(1.1)	0.0	0.0
Ending Balance	$ 2.5	$ 2.5	$ 2.5	$ 2.4	$ 3.5	$ 3.5