Income taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Income taxes
Income tax expense	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Uncertain tax positions	$ 0.1		$ 0.1		$ 0.1