Convertible debt - Schedule Of KSP and Glencore Debt Activity (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2024	Mar. 25, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2024	Sep. 30, 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	$ 342.6			$ 342.6	342.6		$ 288.1
Reconciliation of net change in Convertible debt to Debt extinguishment loss in the nine months ended September 30, 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 nine months ended September 30, 2024					133.9
Proceeds from convertible debt					(75.0)	0.0
Debt extinguishment loss (Note 11)	0.0		$ 0.0		58.9	0.0
KSP Convertible Notes
Conversion Feature Of Convertible Debt [Roll Forward]
Conversion feature at beginning of period		$ 0.0			0.0	6.0	6.0
Fair value (gain) loss on embedded derivative	(0.1)				(0.1)	(4.1)	(6.0)
Conversion feature at end of period	0.0			0.0	0.0		0.0
Debt Component [Roll Forward]
Debt component at beginning of the period		99.1			99.1	85.4	85.4
Issuance of debt component					7.2		9.1
Accrued interest paid in kind					(7.2)		(9.1)
Accrued interest expense					15.0		13.7
Debt component at end of period	114.1			114.1	114.1		99.1
Total Glencore convertible debt at end of period	114.1			114.1	114.1		99.1
Reconciliation of net change in Convertible debt to Debt extinguishment loss in the nine months ended September 30, 2024
Issuance of debt component					7.2		9.1
KSP Convertible Notes | Convertible Debt
Principal Of Convertible Note [Roll Forward]
Principal of convertible note at beginning of period		119.3			119.3	110.2	110.2
Issuance of convertible notes					7.2		9.1
Principal of convertible notes at end of the period	126.5			126.5	126.5		119.3
Reconciliation of net change in Convertible debt to Debt extinguishment loss in the nine months ended September 30, 2024
Proceeds from convertible debt					(7.2)		(9.1)
Glencore Convertible Notes
Principal Of Convertible Note [Roll Forward]
Principal of convertible notes at end of the period	306.2			306.2	306.2
Conversion Feature Of Convertible Debt [Roll Forward]
Conversion feature at beginning of period		0.4			0.4	16.5	16.5
Fair value (gain) loss on embedded derivative	(99.2)		$ (19.3)	(111.9)	(110.1)	(13.4)	(16.1)
Conversion feature at end of period	46.3			46.3	46.3		0.4
Debt Component [Roll Forward]
Debt component at beginning of the period		188.6			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					18.4		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	182.2			182.2	182.2		188.6
Total Glencore convertible debt at end of period	228.5			228.5	228.5		189.0
Reconciliation of net change in Convertible debt to Debt extinguishment loss in the nine months ended September 30, 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			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			300.3	300.3		225.3
Reconciliation of net change in Convertible debt to Debt extinguishment loss in the nine months ended September 30, 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 derivative		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 nine months ended September 30, 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 nine months ended September 30, 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 note at beginning of period				75.0
Principal of convertible notes at end of the period	$ 75.0	$ 75.0		$ 75.0	$ 75.0