v3.25.2
Fair value - Schedule of Fair Value Assets Measured on Recurring Basis (Details) - USD ($)
$ in Thousands
Jun. 30, 2025
Dec. 31, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities $ 5,896,818 $ 5,624,773
Short-term investments 3,001 3,367
State and political subdivisions    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 479,355 467,476
Non-U.S. government    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 154,536 83,802
U.S. corporate    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 2,851,475 2,825,679
Non-U.S. corporate    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 783,184 772,624
Residential mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 261,415 8,364
Commercial mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 48,809  
Other asset-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 1,053,063 1,189,465
Fair value, recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 5,896,818 5,624,773
Short-term investments 3,001 3,367
Total 5,899,819 5,628,140
Fair value, recurring | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 0 0
Short-term investments 0 0
Total 0 0
Fair value, recurring | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 5,640,708 5,343,232
Short-term investments 3,001 3,367
Total 5,643,709 5,346,599
Fair value, recurring | Level 3     
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 256,110 281,541
Short-term investments 0 0
Total 256,110 281,541
Fair value, recurring | U.S. government, agencies and GSEs    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 264,981 277,363
Fair value, recurring | U.S. government, agencies and GSEs | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 0 0
Fair value, recurring | U.S. government, agencies and GSEs | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 264,981 277,363
Fair value, recurring | U.S. government, agencies and GSEs | Level 3     
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 0 0
Fair value, recurring | State and political subdivisions    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 479,355 467,476
Fair value, recurring | State and political subdivisions | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 0 0
Fair value, recurring | State and political subdivisions | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 479,355 467,476
Fair value, recurring | State and political subdivisions | Level 3     
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 0 0
Fair value, recurring | Non-U.S. government    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 154,536 83,802
Fair value, recurring | Non-U.S. government | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 0 0
Fair value, recurring | Non-U.S. government | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 154,536 83,802
Fair value, recurring | Non-U.S. government | Level 3     
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 0 0
Fair value, recurring | U.S. corporate    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 2,851,475 2,825,679
Fair value, recurring | U.S. corporate | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 0 0
Fair value, recurring | U.S. corporate | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 2,644,758 2,602,893
Fair value, recurring | U.S. corporate | Level 3     
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 206,717 222,786
Fair value, recurring | Non-U.S. corporate    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 783,184 772,624
Fair value, recurring | Non-U.S. corporate | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 0 0
Fair value, recurring | Non-U.S. corporate | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 737,081 716,071
Fair value, recurring | Non-U.S. corporate | Level 3     
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 46,103 56,553
Fair value, recurring | Residential mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 261,415 8,364
Fair value, recurring | Residential mortgage-backed | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 0 0
Fair value, recurring | Residential mortgage-backed | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 261,415 8,364
Fair value, recurring | Residential mortgage-backed | Level 3     
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 0 0
Fair value, recurring | Commercial mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 48,809  
Fair value, recurring | Commercial mortgage-backed | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 0  
Fair value, recurring | Commercial mortgage-backed | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 48,809  
Fair value, recurring | Commercial mortgage-backed | Level 3     
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 0  
Fair value, recurring | Other asset-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 1,053,063 1,189,465
Fair value, recurring | Other asset-backed | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 0 0
Fair value, recurring | Other asset-backed | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities 1,049,773 1,187,263
Fair value, recurring | Other asset-backed | Level 3     
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity securities $ 3,290 $ 2,202