Convertible debt - Narrative (Details) $ / shares in Units, $ in Millions				3 Months Ended			6 Months Ended		12 Months Ended
	Mar. 25, 2024 USD ($) tranche $ / shares	Sep. 29, 2021 USD ($) day $ / shares	Sep. 28, 2021 $ / shares	Jun. 30, 2024 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)	Mar. 24, 2024 $ / shares	Mar. 11, 2024 USD ($)	Dec. 31, 2022 USD ($)	May 31, 2022 USD ($) $ / shares
Debt Instrument [Line Items]
Conversion feature of convertible debt (refer to Note 11 (Convertible debt))				$ 145.5	$ 145.5		$ 145.5		$ 0.4
Debt extinguishment loss (Note 11)				0.0		$ 0.0	58.9	$ 0.0
Initial KSP Note | Convertible Debt
Debt Instrument [Line Items]
Amount Issued		$ 100.0		100.0	100.0		100.0
KSP Convertible Notes
Debt Instrument [Line Items]
Conversion feature of convertible debt (refer to Note 11 (Convertible debt))		27.7
Debt instrument, liability component of convertible debt		72.3		107.9	107.9		107.9		99.1			$ 85.4
Debt issuance costs, net		$ 1.6
Embedded derivative, gain on embedded derivative, net				0.1			0.1	0.7	6.0
KSP Convertible Notes | Convertible Debt
Debt Instrument [Line Items]
Amount Issued				126.5	126.5		126.5		119.3			110.2
Debt instrument, interest rate, effective percentage		18.70%
Debt instrument, convertible, conversion price (in dollars per share) | $ / shares		$ 107.44	$ 13.43
Debt instrument, convertible, stock price trigger (in dollars per share) | $ / shares		$ 139.68	$ 17.46
Debt instrument, convertible, threshold trading days | day		20
Debt instrument, redemption price, percentage		130.00%
KSP Convertible Notes | Convertible Debt | London Interbank Offered Rate (LIBOR) | Minimum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	1.00%
KSP Convertible Notes | Convertible Debt | London Interbank Offered Rate (LIBOR) | Maximum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	2.00%
KSP Convertible Notes | Convertible Debt | London Interbank Offered Rate and Secured Overnight Financing Rate | Minimum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate						1.00%
KSP Convertible Notes | Convertible Debt | London Interbank Offered Rate and Secured Overnight Financing Rate | Maximum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate						2.00%
KSP Convertible Notes | Convertible Debt | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		0.58%
KSP Convertible Notes | Convertible Debt | Variable Rate Component One | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		5.00%
KSP Convertible Notes | Convertible Debt | Variable Rate Component Two | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		6.00%
KSP Convertible Notes | Convertible Debt | Variable Rate Component Two | London Interbank Offered Rate (LIBOR) | Minimum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		1.00%
KSP Convertible Notes | Convertible Debt | Variable Rate Component Two | London Interbank Offered Rate (LIBOR) | Maximum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		2.00%
First A&R Glencore Note | Convertible Debt
Debt Instrument [Line Items]
Amount Issued				116.6	116.6		116.6
Debt instrument, modification date, term after initial funding	1 month
Debt instrument, maturity, additional term	5 years
A&R Convertible Notes | Convertible Debt
Debt Instrument [Line Items]
Debt instrument, convertible, conversion price (in dollars per share) | $ / shares	$ 79.60									$ 9.95
Debt instrument, convertible, conversion price adjustment, premium, percentage	25.00%
Debt instrument, convertible, conversion price adjustment, volume weighted average trading price, duration from modification date	30 days
Debt extinguishment loss (Note 11)				58.9			58.9
A&R Convertible Notes And Senior Secured Convertible Glencore Notes | Convertible Debt
Debt Instrument [Line Items]
Debt instrument, convertible, conversion price (in dollars per share) | $ / shares													$ 0.206
Senior Secured Convertible Glencore Note | Convertible Debt
Debt Instrument [Line Items]
Amount Issued	$ 75.0			75.0	75.0		75.0
Debt instrument, convertible, conversion price (in dollars per share) | $ / shares	$ 4.24									$ 0.53
Senior Secured Convertible Glencore Note | Convertible Debt | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	1.00%
Senior Secured Convertible Glencore Note | Convertible Debt | Variable Rate Component One | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	5.00%
Senior Secured Convertible Glencore Note | Convertible Debt | Variable Rate Component Two | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	6.00%
Senior Secured Convertible Glencore Note | Convertible Debt | Variable Rate Component Three | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	1.00%
Glencore Convertible Notes
Debt Instrument [Line Items]
Amount Issued				306.2	306.2		306.2
Conversion feature of convertible debt (refer to Note 11 (Convertible debt))													$ 46.2
Debt instrument, liability component of convertible debt				173.0	173.0		173.0		188.6			164.9	153.8
Debt issuance costs, net													$ 1.3
Embedded derivative, gain on embedded derivative, net				34.7	12.7	$ 6.4	10.9	$ 5.9	16.1
Glencore Convertible Notes | Convertible Debt
Debt Instrument [Line Items]
Amount Issued				$ 300.3	$ 300.3		$ 300.3		$ 225.3		$ 75.0	$ 208.1
Glencore Unsecured Convertible Notes | Convertible Debt
Debt Instrument [Line Items]
Debt instrument, number of tranches | tranche	2