v3.26.1
Fair value - Summary of Significant Inputs used for Fair Value Measurement of Fixed Maturity Securities (Details) - USD ($)
$ in Thousands
Jun. 30, 2026
Dec. 31, 2025
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value $ 6,161,975 $ 6,050,542
State and political subdivisions    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 466,452 478,972
Non-U.S. government    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 198,849 185,462
U.S. corporate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 2,776,874 2,810,727
Non-U.S. corporate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 849,991 783,056
Residential mortgage-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 358,002 349,333
Commercial mortgage-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 223,468 129,562
Other asset-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 1,002,197 1,056,123
Fair value, recurring    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 6,161,975 6,050,542
Fair value, recurring | U.S. government, agencies and GSEs    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 286,142 257,307
Fair value, recurring | State and political subdivisions    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 466,452 478,972
Fair value, recurring | Non-U.S. government    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 198,849 185,462
Fair value, recurring | U.S. corporate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 2,776,874 2,810,727
Fair value, recurring | Non-U.S. corporate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 849,991 783,056
Fair value, recurring | Residential mortgage-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 358,002 349,333
Fair value, recurring | Commercial mortgage-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 223,468 129,562
Fair value, recurring | Other asset-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 1,002,197 1,056,123
Level 2 | Fair value, recurring    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 5,899,134 5,791,629
Level 2 | Fair value, recurring | U.S. government, agencies and GSEs    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 286,142 257,307
Level 2 | Fair value, recurring | State and political subdivisions    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 466,452 478,972
Level 2 | Fair value, recurring | Non-U.S. government    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 198,849 185,462
Level 2 | Fair value, recurring | U.S. corporate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 2,594,154 2,613,006
Level 2 | Fair value, recurring | Non-U.S. corporate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 814,309 748,645
Level 2 | Fair value, recurring | Residential mortgage-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 352,188 348,850
Level 2 | Fair value, recurring | Commercial mortgage-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 216,055 122,095
Level 2 | Fair value, recurring | Other asset-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 970,985 $ 1,037,292
Pricing Services | Level 2 | Fair value, recurring | U.S. government, agencies and GSEs    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 286,142  
Pricing Services | Level 2 | Fair value, recurring | State and political subdivisions    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 466,452  
Pricing Services | Level 2 | Fair value, recurring | Non-U.S. government    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 198,849  
Pricing Services | Level 2 | Fair value, recurring | U.S. corporate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 2,366,458  
Pricing Services | Level 2 | Fair value, recurring | Non-U.S. corporate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 728,719  
Pricing Services | Level 2 | Fair value, recurring | Residential mortgage-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 352,188  
Pricing Services | Level 2 | Fair value, recurring | Commercial mortgage-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 216,055  
Pricing Services | Level 2 | Fair value, recurring | Other asset-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value $ 970,985