Convertible debt - Schedule Of KSP and Glencore Debt Activity (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2024	Dec. 31, 2023
Principal Of Convertible Note [Roll Forward]
Issuance of convertible notes		$ 75.0
Debt Component [Roll Forward]
Total convertible debt at end of period	$ 363.1	363.1	$ 288.1
Reconciliation of net change in Convertible debt to Debt extinguishment loss in the year ended December 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 year ended December 31, 2024		133.9
Proceeds from convertible debt		(75.0)
Debt extinguishment loss		58.9	0.0
KSP Convertible Notes
Principal Of Convertible Note [Roll Forward]
Principal of convertible notes at end of the period	327.4	327.4
Conversion Feature Of Convertible Debt [Roll Forward]
Conversion feature at beginning of period		0.0	6.0
Fair value gain on embedded derivative		0.0	(6.0)
Conversion feature issued		0.0	0.0
Conversion feature at end of period	0.0	0.0	0.0
Debt Component [Roll Forward]
Debt component at beginning of the period		99.1	85.4
Issuance of debt component		14.4	9.1
Accrued interest paid in kind		(14.4)	(9.1)
Accrued interest expense		20.2	13.7
Transaction costs		0.0	0.0
Debt component at end of period	119.3	119.3	99.1
Total convertible debt at end of period	119.3	119.3	99.1
Reconciliation of net change in Convertible debt to Debt extinguishment loss in the year ended December 31, 2024
Conversion feature issued		0.0	0.0
Issuance of debt component		14.4	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
Issuance of convertible notes		14.4	9.1
Principal of convertible notes at end of the period	133.7	133.7	119.3
Reconciliation of net change in Convertible debt to Debt extinguishment loss in the year ended December 31, 2024
Proceeds from convertible debt		(14.4)	(9.1)
Glencore Convertible Notes
Conversion Feature Of Convertible Debt [Roll Forward]
Conversion feature at beginning of period		0.4	16.5
Fair value gain on embedded derivative	(106.9)	(105.1)	(16.1)
Conversion feature at end of period	51.3	51.3	0.4
Debt Component [Roll Forward]
Debt component at beginning of the period	153.8	188.6	164.9
Issuance of debt component		21.2	17.2
Accrued interest paid in kind		(21.2)	(17.2)
Accrued interest expense		28.7	23.7
Accrued interest and accretion expense from January 1, 2024 to March 25, 2024		5.9	0.0
Transaction costs		(8.6)	0.0
Debt component at end of period	192.5	192.5	188.6
Total convertible debt at end of period	243.8	243.8	189.0
Reconciliation of net change in Convertible debt to Debt extinguishment loss in the year ended December 31, 2024
Issuance of debt component		21.2	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
Issuance of convertible notes		102.2	17.2
Principal of convertible notes at end of the period	327.5	327.5	225.3
Reconciliation of net change in Convertible debt to Debt extinguishment loss in the year ended December 31, 2024
Proceeds from convertible debt		(102.2)	(17.2)
A&R Glencore Convertible Notes
Conversion Feature Of Convertible Debt [Roll Forward]
Fair value gain on embedded derivative		1.8
Extinguishment of the conversion feature embedded in the A&R Glencore Convertible Notes as part of the modification		(2.2)
Conversion feature issued		99.2
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)
Reconciliation of net change in Convertible debt to Debt extinguishment loss in the year ended December 31, 2024
Extinguishment of the conversion feature embedded in the A&R Glencore Convertible Notes as part of the modification		(2.2)
Conversion feature issued		99.2
Issuance of debt component		124.4
Senior Secured Convertible Glencore Note
Conversion Feature Of Convertible Debt [Roll Forward]
Conversion feature issued		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 year ended December 31, 2024
Conversion feature issued		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 note at beginning of period	75.0
Principal of convertible notes at end of the period	$ 75.0	75.0
A&R Convertible Notes | Convertible Debt
Reconciliation of net change in Convertible debt to Debt extinguishment loss in the year ended December 31, 2024
Debt extinguishment loss		$ 58.9