v3.26.3
Investments - Summary of Securities with Gross Unrealized Loss Positions Aggregated by Investment Category (Detail) - USD ($)
$ in Thousands
Sep. 30, 2026
Dec. 31, 2025
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Losses, Less than Twelve Months $ (53,096) $ (1,117)
Estimated Fair Value, Less than Twelve Months 1,225,780 253,325
Gross Unrealized Losses, Twelve Months or Longer (479) (259)
Estimated Fair Value, Twelve Months or Longer 5,256 4,105
Gross Unrealized Losses, Total (53,575) (1,376)
Estimated Fair Value, Total 1,231,036 257,430
Fixed-Maturity Securities [Member] | U.S. Treasury and U.S. Government Agencies [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Losses, Less than Twelve Months (46,650) (553)
Estimated Fair Value, Less than Twelve Months 978,082 208,273
Gross Unrealized Losses, Twelve Months or Longer (185) (164)
Estimated Fair Value, Twelve Months or Longer 2,894 2,918
Gross Unrealized Losses, Total (46,835) (717)
Estimated Fair Value, Total 980,976 211,191
Fixed-Maturity Securities [Member] | Corporate Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Losses, Less than Twelve Months (5,819) (289)
Estimated Fair Value, Less than Twelve Months 231,637 28,303
Gross Unrealized Losses, Twelve Months or Longer (244) (95)
Estimated Fair Value, Twelve Months or Longer 1,112 1,187
Gross Unrealized Losses, Total (6,063) (384)
Estimated Fair Value, Total 232,749 29,490
Fixed-Maturity Securities [Member] | Commercial Mortgage Backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Losses, Less than Twelve Months (627) (275)
Estimated Fair Value, Less than Twelve Months 16,061 16,749
Gross Unrealized Losses, Twelve Months or Longer (50) 0
Estimated Fair Value, Twelve Months or Longer 1,250 0
Gross Unrealized Losses, Total (677) (275)
Estimated Fair Value, Total $ 17,311 $ 16,749