v3.26.1
Fair value measurements - Schedule of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis by Level (Details) - USD ($)
$ in Millions
Jun. 30, 2026
Dec. 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value $ 662.2 $ 712.0
Corporate    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 228.8 246.9
US government and agency    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 130.6 124.5
Non-US government and agency    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 225.7 248.1
Residential mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 54.1 55.6
Commercial mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 7.7 15.0
Other asset-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 15.3 21.9
Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 662.2 712.0
Recurring | Corporate    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 228.8 246.9
Recurring | US government and agency    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 130.6 124.5
Recurring | Non-US government and agency    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 225.7 248.1
Recurring | Residential mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 54.1 55.6
Recurring | Commercial mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 7.7 15.0
Recurring | Other asset-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 15.3 21.9
Recurring | Quoted prices in active markets for identical assets Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 0.0 0.0
Recurring | Quoted prices in active markets for identical assets Level 1 | Corporate    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 0.0 0.0
Recurring | Quoted prices in active markets for identical assets Level 1 | US government and agency    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 0.0 0.0
Recurring | Quoted prices in active markets for identical assets Level 1 | Non-US government and agency    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 0.0 0.0
Recurring | Quoted prices in active markets for identical assets Level 1 | Residential mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 0.0 0.0
Recurring | Quoted prices in active markets for identical assets Level 1 | Commercial mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 0.0 0.0
Recurring | Quoted prices in active markets for identical assets Level 1 | Other asset-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 0.0 0.0
Recurring | Significant other observable Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 662.2 712.0
Recurring | Significant other observable Level 2 | Corporate    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 228.8 246.9
Recurring | Significant other observable Level 2 | US government and agency    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 130.6 124.5
Recurring | Significant other observable Level 2 | Non-US government and agency    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 225.7 248.1
Recurring | Significant other observable Level 2 | Residential mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 54.1 55.6
Recurring | Significant other observable Level 2 | Commercial mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 7.7 15.0
Recurring | Significant other observable Level 2 | Other asset-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 15.3 21.9
Recurring | Significant unobservable inputs Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 0.0 0.0
Recurring | Significant unobservable inputs Level 3 | Corporate    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 0.0 0.0
Recurring | Significant unobservable inputs Level 3 | US government and agency    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 0.0 0.0
Recurring | Significant unobservable inputs Level 3 | Non-US government and agency    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 0.0 0.0
Recurring | Significant unobservable inputs Level 3 | Residential mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 0.0 0.0
Recurring | Significant unobservable inputs Level 3 | Commercial mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 0.0 0.0
Recurring | Significant unobservable inputs Level 3 | Other asset-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value $ 0.0 $ 0.0