v3.26.1
Fair value measurements - Schedule of Assets Measured at Fair Value on Non-Recurring Basis (Details) - USD ($)
$ in Millions
Jun. 30, 2026
Dec. 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total $ 88.1 $ 84.0
MGAs and TPAs    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total 78.9 59.9
Venture funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total 9.2 24.1
Nonrecurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total 88.1 84.0
Nonrecurring | MGAs and TPAs    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total 78.9 59.9
Nonrecurring | Venture funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total 9.2 24.1
Nonrecurring | Quoted prices in active markets for identical assets Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total 0.0 0.0
Nonrecurring | Quoted prices in active markets for identical assets Level 1 | MGAs and TPAs    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total 0.0 0.0
Nonrecurring | Quoted prices in active markets for identical assets Level 1 | Venture funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total 0.0 0.0
Nonrecurring | Significant other observable Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total 0.0 0.0
Nonrecurring | Significant other observable Level 2 | MGAs and TPAs    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total 0.0 0.0
Nonrecurring | Significant other observable Level 2 | Venture funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total 0.0 0.0
Nonrecurring | Significant unobservable inputs Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total 88.1 84.0
Nonrecurring | Significant unobservable inputs Level 3 | MGAs and TPAs    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total 78.9 59.9
Nonrecurring | Significant unobservable inputs Level 3 | Venture funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total $ 9.2 $ 24.1