Financial assets and liabilities - Assets and Liabilities Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts receivable (subject to provisional pricing)	$ 0.4	$ 0.6
Conversion feature of convertible debt (refer to Note 11 (Convertible debt))	145.5	0.4
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts receivable (subject to provisional pricing)	0.0	0.0
Conversion feature of convertible debt (refer to Note 11 (Convertible debt))	0.0	0.0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts receivable (subject to provisional pricing)	0.4	0.6
Conversion feature of convertible debt (refer to Note 11 (Convertible debt))	$ 145.5	$ 0.4