Income taxes - Reconciliation of unrecognized tax benefits (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2023	Mar. 31, 2023
Income Tax Disclosure
Beginning Balance	$ 1.5	$ 1.5	$ 2.5	$ 2.5
Reduction as a result of lapse of statute of limitations	0.0	0.0	0.0	0.0
Ending Balance	$ 1.5	$ 1.5	$ 2.5	$ 2.5