v3.26.3
Fair value measurements - Financial Instruments not Measured at Fair Value (Details) - Level 2 - USD ($)
$ in Millions
Sep. 30, 2026
Dec. 31, 2025
Fair Value | Series B preference shares    
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]    
Series B preference shares $ 0.0 $ 202.2
Fair Value | 2017 SEK Subordinated Notes    
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]    
Long-term debt 276.0 293.3
Carrying Value | Series B preference shares    
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]    
Series B preference shares 0.0 200.0
Carrying Value | 2017 SEK Subordinated Notes    
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]    
Long-term debt 270.6 292.6
2024 Senior Notes | Fair Value | Senior Notes    
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]    
Long-term debt 406.8 424.2
2024 Senior Notes | Carrying Value | Senior Notes    
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]    
Long-term debt $ 396.9 $ 396.0