Convertible debt - Schedule Of KSP and Glencore Debt Activity (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Principal Of Convertible Note [Roll Forward]
Issuance of convertible notes	$ 75.0	$ 0.0
Debt Component [Roll Forward]
Total Glencore convertible debt at end of period	447.7		$ 288.1
Reconciliation of net change in Convertible debt to Debt extinguishment loss in the three months ended March 31, 2024
Total change in the conversion features	156.0
Total change in the debt components	(22.1)
Total net change in convertible debt in the three months ended March 31, 2024	133.9
Proceeds from convertible debt	(75.0)	0.0
Debt extinguishment loss (Note 14)	58.9	0.0
KSP Convertible Notes
Conversion Feature Of Convertible Debt [Roll Forward]
Conversion feature at beginning of period	0.0	6.0	6.0
Fair value gain (loss) on embedded derivatives	0.1	0.2	(6.0)
Conversion feature at end of period	0.1		0.0
Debt Component [Roll Forward]
Debt component at beginning of the period	99.1	85.4	85.4
Issuance of debt component	0.0		9.1
Accrued interest paid in kind	0.0		(9.1)
Accrued interest expense	3.8		13.7
Debt component at end of period	102.9		99.1
Total Glencore convertible debt at end of period	103.0		99.1
Reconciliation of net change in Convertible debt to Debt extinguishment loss in the three months ended March 31, 2024
Issuance of debt component	0.0		9.1
KSP Convertible Notes | Convertible Debt
Principal Of Convertible Note [Roll Forward]
Principal of convertible note at beginning of period	119.3	110.2	110.2
Issuance of convertible notes	0.0		9.1
Principal of convertible notes at end of the period	119.3		119.3
Reconciliation of net change in Convertible debt to Debt extinguishment loss in the three months ended March 31, 2024
Proceeds from convertible debt	0.0		(9.1)
Glencore Convertible Notes
Principal Of Convertible Note [Roll Forward]
Principal of convertible notes at end of the period	306.2
Conversion Feature Of Convertible Debt [Roll Forward]
Conversion feature at beginning of period	0.4	16.5	16.5
Fair value gain (loss) on embedded derivatives	21.9	0.5	(16.1)
Conversion feature at end of period	180.1		0.4
Debt Component [Roll Forward]
Debt component at beginning of the period	188.6	164.9	164.9
Issuance of debt component	0.0		17.2
Accrued interest paid in kind	0.0		(17.2)
Accrued interest expense	0.6		23.7
Accrued interest and accretion expense from January 1, 2024 to March 25, 2024	5.9		0.0
Transaction costs	(8.4)		0.0
Debt component at end of period	164.6		188.6
Total Glencore convertible debt at end of period	344.7		189.0
Reconciliation of net change in Convertible debt to Debt extinguishment loss in the three months ended March 31, 2024
Issuance of debt component	0.0		17.2
Glencore Convertible Notes | Convertible Debt
Principal Of Convertible Note [Roll Forward]
Principal of convertible note at beginning of period	225.3	$ 208.1	208.1
Issuance of convertible notes	75.0		17.2
Principal of convertible notes at end of the period	300.3		225.3
Reconciliation of net change in Convertible debt to Debt extinguishment loss in the three months ended March 31, 2024
Proceeds from convertible debt	(75.0)		(17.2)
A&R Glencore Convertible Notes
Conversion Feature Of Convertible Debt [Roll Forward]
Fair value gain (loss) on embedded derivatives	1.8		0.0
Extinguishment of the conversion feature embedded in the A&R Glencore Convertible Notes as part of the modification	(2.2)		0.0
Issuance of conversion feature	99.2		0.0
Debt Component [Roll Forward]
Issuance of debt component	124.4
Extinguishment of the debt component related to A&R Glencore Convertible Notes as part of the modification	(194.5)		0.0
Reconciliation of net change in Convertible debt to Debt extinguishment loss in the three months ended March 31, 2024
Extinguishment of the conversion feature embedded in the A&R Glencore Convertible Notes as part of the modification	(2.2)
Issuance of conversion feature	99.2		0.0
Extinguishment of the debt component related to A&R Glencore Convertible Notes as part of the modification	(194.5)		0.0
Issuance of debt component	124.4
Senior Secured Convertible Glencore Note
Conversion Feature Of Convertible Debt [Roll Forward]
Issuance of conversion feature	59.0		0.0
Debt Component [Roll Forward]
Issuance of debt component	48.0		0.0
Reconciliation of net change in Convertible debt to Debt extinguishment loss in the three months ended March 31, 2024
Issuance of conversion feature	59.0		0.0
Issuance of debt component	48.0		$ 0.0
Senior Secured Convertible Glencore Note | Convertible Debt
Principal Of Convertible Note [Roll Forward]
Principal of convertible notes at end of the period	$ 75.0