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Convertible debt - Narrative (Details)
$ / shares in Units, $ in Millions
3 Months Ended 6 Months Ended 9 Months Ended 12 Months Ended
Jun. 03, 2024
$ / shares
Jun. 02, 2024
$ / shares
Mar. 25, 2024
USD ($)
tranche
$ / shares
Sep. 29, 2021
USD ($)
day
$ / shares
Sep. 30, 2024
USD ($)
shares
Mar. 31, 2024
USD ($)
Sep. 30, 2023
USD ($)
Jun. 30, 2023
Sep. 30, 2024
USD ($)
Sep. 30, 2024
USD ($)
Sep. 30, 2023
USD ($)
Dec. 31, 2023
USD ($)
Mar. 11, 2024
USD ($)
Dec. 31, 2022
USD ($)
May 31, 2022
USD ($)
$ / shares
Debt Instrument [Line Items]                              
Conversion feature of convertible debt (refer to Note 11 (Convertible debt))         $ 46.3       $ 46.3 $ 46.3   $ 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       72.3 114.1       114.1 114.1   99.1   $ 85.4  
Debt issuance costs, net       $ 1.6                      
Embedded derivative, gain on embedded derivative, net         0.1         0.1 4.1 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 $ 106.73 $ 13.34   $ 107.44                      
Debt conversion, converted instrument, shares issued (in shares) | shares         701,323                    
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%                      
Glencore Unsecured Convertible Notes | Convertible Debt                              
Debt Instrument [Line Items]                              
Debt instrument, number of tranches | tranche     2                        
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 79.09 9.89 $ 79.60                        
Debt instrument, convertible, conversion price adjustment, volume weighted average trading price, duration from modification date     30 days                        
Debt instrument, convertible, conversion price adjustment, premium, percentage     25.00%                        
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 $ 4.23 $ 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         182.2       182.2 182.2   188.6   164.9 153.8
Debt issuance costs, net                             $ 1.3
Embedded derivative, gain on embedded derivative, net         99.2   $ 19.3   111.9 110.1 $ 13.4 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