Income taxes - Reconciliation of unrecognized tax benefits (Details) - USD ($) $ in Millions	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023
Income Tax Disclosure
Beginning Balance	$ 1.5	$ 1.5	$ 1.5	$ 2.5	$ 2.5	$ 2.5	$ 2.5
Ending Balance	$ 1.5	$ 1.5	$ 1.5	$ 1.5	$ 2.5	$ 2.5	$ 2.5