v3.26.1
Fair value - Summary of Significant Inputs used for Fair Value Measurement of Fixed Maturity Securities (Details) - USD ($)
$ in Thousands
Mar. 31, 2026
Dec. 31, 2025
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value $ 6,133,789 $ 6,050,542
State and political subdivisions    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 469,832 478,972
Non-U.S. government    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 179,543 185,462
U.S. corporate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 2,823,996 2,810,727
Non-U.S. corporate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 816,324 783,056
Residential mortgage-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 341,484 349,333
Commercial mortgage-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 198,632 129,562
Other asset-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 1,035,256 1,056,123
Fair value, recurring    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 6,133,789 6,050,542
Fair value, recurring | U.S. government, agencies and GSEs    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 268,722 257,307
Fair value, recurring | State and political subdivisions    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 469,832 478,972
Fair value, recurring | Non-U.S. government    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 179,543 185,462
Fair value, recurring | U.S. corporate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 2,823,996 2,810,727
Fair value, recurring | Non-U.S. corporate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 816,324 783,056
Fair value, recurring | Residential mortgage-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 341,484 349,333
Fair value, recurring | Commercial mortgage-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 198,632 129,562
Fair value, recurring | Other asset-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 1,035,256 1,056,123
Level 2 | Fair value, recurring    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 5,879,477 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 268,722 257,307
Level 2 | Fair value, recurring | State and political subdivisions    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 469,832 478,972
Level 2 | Fair value, recurring | Non-U.S. government    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 179,543 185,462
Level 2 | Fair value, recurring | U.S. corporate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 2,634,606 2,613,006
Level 2 | Fair value, recurring | Non-U.S. corporate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 782,425 748,645
Level 2 | Fair value, recurring | Residential mortgage-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 341,038 348,850
Level 2 | Fair value, recurring | Commercial mortgage-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 191,214 122,095
Level 2 | Fair value, recurring | Other asset-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 1,012,097 $ 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 268,722  
Pricing Services | Level 2 | Fair value, recurring | State and political subdivisions    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 469,832  
Pricing Services | Level 2 | Fair value, recurring | Non-U.S. government    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 179,543  
Pricing Services | Level 2 | Fair value, recurring | U.S. corporate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 2,404,183  
Pricing Services | Level 2 | Fair value, recurring | Non-U.S. corporate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 699,923  
Pricing Services | Level 2 | Fair value, recurring | Residential mortgage-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 341,038  
Pricing Services | Level 2 | Fair value, recurring | Commercial mortgage-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 191,214  
Pricing Services | Level 2 | Fair value, recurring | Other asset-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value $ 1,012,097