Convertible debt - Schedule of KSP PIK Notes (Details) - Convertible Debt - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Sep. 29, 2021
KSP Convertible Notes
Debt Conversion [Line Items]
Amount Issued	$ 119.3	$ 119.3	$ 110.2
Initial KSP Note
Debt Conversion [Line Items]
Amount Issued	100.0			$ 100.0
KSP PIK Note, Issued December 31, 2021
Debt Conversion [Line Items]
Amount Issued	1.8
KSP PIK Note, Issued June 30, 2022
Debt Conversion [Line Items]
Amount Issued	4.1
KSP PIK Note, Issued December 31, 2022
Debt Conversion [Line Items]
Amount Issued	4.3
KSP PIK Note, Issued June 30, 2023
Debt Conversion [Line Items]
Amount Issued	4.4
KSP PIK Note, Issued December 31, 2023
Debt Conversion [Line Items]
Amount Issued	$ 4.7