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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, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Segment Reporting Information [Line Items] |  |  |
Premiums earned | $ 7,036 | $ 6,531 | $ 6,085
Losses and loss adjustment expenses | 4,460 | 4,033 | 3,629
Commissions and other underwriting expenses | 2,007 | 1,935 | 1,718
Operating segments | Property and casualty insurance |  |  |
Segment Reporting Information [Line Items] |  |  |
Premiums earned | 7,036 | 6,531 | 6,085
Losses and loss adjustment expenses | 4,455 | 4,017 | 3,629
Commissions and other underwriting expenses | 1,961 | 1,883 | 1,680
Property and casualty insurance underwriting | 620 | 631 | 776
Operating segments | Property and casualty insurance | Property and transportation |  |  |
Segment Reporting Information [Line Items] |  |  |
Premiums earned | 2,793 | 2,519 | 2,487
Losses and loss adjustment expenses | 1,952 | 1,741 | 1,735
Commissions and other underwriting expenses | 630 | 594 | 544
Property and casualty insurance underwriting | 211 | 184 | 208
Operating segments | Property and casualty insurance | Specialty casualty |  |  |
Segment Reporting Information [Line Items] |  |  |
Premiums earned | 2,967 | 2,886 | 2,659
Losses and loss adjustment expenses | 1,853 | 1,739 | 1,453
Commissions and other underwriting expenses | 774 | 772 | 706
Property and casualty insurance underwriting | 340 | 375 | 500
Operating segments | Property and casualty insurance | Specialty financial |  |  |
Segment Reporting Information [Line Items] |  |  |
Premiums earned | 1,032 | 867 | 698
Losses and loss adjustment expenses | 429 | 328 | 238
Commissions and other underwriting expenses | 469 | 429 | 346
Property and casualty insurance underwriting | 134 | 110 | 114
Operating segments | Property and casualty insurance | Other specialty |  |  |
Segment Reporting Information [Line Items] |  |  |
Premiums earned | 244 | 259 | 241
Losses and loss adjustment expenses | 215 | 207 | 199
Commissions and other underwriting expenses | 88 | 88 | 84
Property and casualty insurance underwriting | (59) | (36) | (42)
Operating segments | Property and casualty insurance | Other lines |  |  |
Segment Reporting Information [Line Items] |  |  |
Losses and loss adjustment expenses | 6 | 2 | 4
Property and casualty insurance underwriting | $ (6) | $ (2) | $ (4)

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- Definition Underwriting income (loss) for the property and casualty insurance segment, which is calculated as premiums earned less losses and loss adjustment expenses less commissions and other underwriting expenses. + References No definition available. + Details
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- Definition Amount, after effects of policies assumed or ceded, of expense related to the provision for policy benefits and costs incurred. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 220 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(5)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477250/944-220-S99-1 + Details
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- Definition Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17(Column E)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-2 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (c)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479032/944-605-50-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04(13)(c)) -SubTopic 220 -Topic 942 -Publisher FASB -URI https://asc.fasb.org/1943274/2147478524/942-220-S99-1 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(1)) -SubTopic 220 -Topic 944 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477250/944-220-S99-1 + Details
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- Definition The aggregate total costs related to selling a firm's product and services, as well as all other general and administrative expenses. Direct selling expenses (for example, credit, warranty, and advertising) are expenses that can be directly linked to the sale of specific products. Indirect selling expenses are expenses that cannot be directly linked to the sale of specific products, for example telephone expenses, Internet, and postal charges. General and administrative expenses include salaries of non-sales personnel, rent, utilities, communication, etc. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(4)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147483621/220-10-S99-2 + Details
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