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Fair value measurements - Financial Instruments not Measured at Fair Value (Details) - Level 2 - USD ($)
$ in Millions
Mar. 31, 2025
Dec. 31, 2024
Fair Value | Series B preference shares    
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]    
Series B preference shares $ 201.3 $ 206.0
Fair Value | Senior Notes | 2024 Senior Notes    
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]    
Long-term debt 417.2 411.2
Fair Value | Subordinated Notes    
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]    
Long-term debt 251.8 228.7
Carrying Value | Series B preference shares    
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]    
Series B preference shares 200.0 200.0
Carrying Value | Senior Notes | 2024 Senior Notes    
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]    
Long-term debt 395.1 394.8
Carrying Value | Subordinated Notes    
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]    
Long-term debt $ 268.4 $ 244.3