v3.25.2
Fair value - Summary of Significant Inputs used for Fair Value Measurement of Fixed Maturity Securities (Details) - USD ($)
$ in Thousands
Jun. 30, 2025
Dec. 31, 2024
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value $ 5,896,818 $ 5,624,773
State and political subdivisions    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 479,355 467,476
Non-U.S. government    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 154,536 83,802
U.S. corporate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 2,851,475 2,825,679
Non-U.S. corporate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 783,184 772,624
Residential mortgage-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 261,415 8,364
Commercial mortgage-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 48,809  
Other asset-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 1,053,063 1,189,465
Fair value, recurring    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 5,896,818 5,624,773
Fair value, recurring | U.S. government, agencies and GSEs    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 264,981 277,363
Fair value, recurring | State and political subdivisions    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 479,355 467,476
Fair value, recurring | Non-U.S. government    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 154,536 83,802
Fair value, recurring | U.S. corporate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 2,851,475 2,825,679
Fair value, recurring | Non-U.S. corporate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 783,184 772,624
Fair value, recurring | Residential mortgage-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 261,415 8,364
Fair value, recurring | Commercial mortgage-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 48,809  
Fair value, recurring | Other asset-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 1,053,063 1,189,465
Level 2 | Fair value, recurring    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 5,640,708 5,343,232
Level 2 | Fair value, recurring | U.S. government, agencies and GSEs    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 264,981 277,363
Level 2 | Fair value, recurring | State and political subdivisions    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 479,355 467,476
Level 2 | Fair value, recurring | Non-U.S. government    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 154,536 83,802
Level 2 | Fair value, recurring | U.S. corporate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 2,644,758 2,602,893
Level 2 | Fair value, recurring | Non-U.S. corporate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 737,081 716,071
Level 2 | Fair value, recurring | Residential mortgage-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 261,415 8,364
Level 2 | Fair value, recurring | Commercial mortgage-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 48,809  
Level 2 | Fair value, recurring | Other asset-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 1,049,773 $ 1,187,263
Pricing Services | Level 2 | Fair value, recurring | U.S. government, agencies and GSEs    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 264,981  
Pricing Services | Level 2 | Fair value, recurring | State and political subdivisions    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 479,355  
Pricing Services | Level 2 | Fair value, recurring | Non-U.S. government    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 154,536  
Pricing Services | Level 2 | Fair value, recurring | U.S. corporate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 2,427,112  
Pricing Services | Level 2 | Fair value, recurring | Non-U.S. corporate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 659,665  
Pricing Services | Level 2 | Fair value, recurring | Residential mortgage-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 261,415  
Pricing Services | Level 2 | Fair value, recurring | Commercial mortgage-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value 48,809  
Pricing Services | Level 2 | Fair value, recurring | Other asset-backed    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value $ 1,049,773