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Segments of Operations - Components of underwriting profit, including significant segment expenses by segment and sub-segment (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Segment Reporting Information [Line Items]      
Net earned premiums $ 7,046 $ 7,036 $ 6,531
Losses and loss adjustment expenses 4,388 4,460 4,033
Commissions and other underwriting expenses 2,059 2,007 1,935
Operating segments | Property and casualty insurance      
Segment Reporting Information [Line Items]      
Net earned premiums 7,046 7,036 6,531
Losses and loss adjustment expenses 4,388 4,455 4,017
Commissions and other underwriting expenses 2,029 1,961 1,883
Property and casualty insurance underwriting 629 620 631
Operating segments | Property and casualty insurance | Property and transportation      
Segment Reporting Information [Line Items]      
Net earned premiums 2,746 2,826 2,550
Losses and loss adjustment expenses 1,843 1,972 1,773
Commissions and other underwriting expenses 568 640 603
Property and casualty insurance underwriting 335 214 174
Operating segments | Property and casualty insurance | Specialty casualty      
Segment Reporting Information [Line Items]      
Net earned premiums 3,215 3,176 3,112
Losses and loss adjustment expenses 2,151 2,045 1,914
Commissions and other underwriting expenses 935 852 850
Property and casualty insurance underwriting 129 279 348
Operating segments | Property and casualty insurance | Specialty financial      
Segment Reporting Information [Line Items]      
Net earned premiums 1,085 1,034 869
Losses and loss adjustment expenses 389 432 328
Commissions and other underwriting expenses 526 469 430
Property and casualty insurance underwriting 170 133 111
Operating segments | Property and casualty insurance | Other lines      
Segment Reporting Information [Line Items]      
Losses and loss adjustment expenses 5 6 2
Property and casualty insurance underwriting $ (5) $ (6) $ (2)