Convertible debt - Schedule of Total Convertible Debt (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Debt Conversion [Line Items]
Convertible debt	$ 342.6	$ 288.1
KSP Convertible Notes
Debt Conversion [Line Items]
Convertible debt	114.1	99.1
Glencore Convertible Notes
Debt Conversion [Line Items]
Convertible debt	$ 228.5	$ 189.0

Carrying amount of long-term convertible debt as of the balance sheet date, net of the amount due in the next twelve months or greater than the normal operating cycle, if longer. The debt is convertible into another form of financial instrument, typically the entity's common stock.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-SubTopic 10
-Topic 210
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Name:	us-gaap_ConvertibleDebtNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant