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Segment Information - Summary of net premiums written and earned by major line of business and net premiums written by location (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | $ 15,100 | $ 12,440 | $ 9,679
Net premiums written | 15,732 | 13,468 | 11,078
Operating segments | Insurance |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 6,627 | 5,446 | 4,560
Net premiums written | 6,874 | 5,862 | 5,021
Operating segments | Insurance | North America |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 4,702 | 3,781 | 3,283
Net premiums written | 4,869 | 3,995 | 3,527
Operating segments | Insurance | International |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 1,925 | 1,665 | 1,277
Net premiums written | 2,005 | 1,867 | 1,494
Operating segments | Insurance | Property and short-tail specialty | North America |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 1,165 | 976 | 746
Operating segments | Insurance | Property and short-tail specialty | International |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 1,050 | 885 | 613
Operating segments | Insurance | Casualty and other | International |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 875 | 780 | 664
Operating segments | Insurance | Other liability - occurrence | North America |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 942 | 618 | 509
Operating segments | Insurance | Other liability - claims made | North America |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 843 | 866 | 874
Operating segments | Insurance | Workers compensation | North America |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 549 | 495 | 415
Operating segments | Insurance | Commercial automobile | North America |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 459 | 343 | 287
Operating segments | Insurance | Commercial multi-peril | North America |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 435 | 193 | 197
Operating segments | Insurance | Other | North America |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 309 | 290 | 255
Operating segments | Reinsurance |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 7,242 | 5,836 | 3,959
Net premiums written | 7,746 | 6,554 | 4,924
Operating segments | Reinsurance | Bermuda |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums written | 3,425 | 3,288 | 2,561
Operating segments | Reinsurance | United States |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums written | 2,135 | 1,756 | 1,247
Operating segments | Reinsurance | Europe and other |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums written | 2,186 | 1,510 | 1,116
Operating segments | Reinsurance | Other |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 152 | 118 | 110
Operating segments | Reinsurance | Other specialty |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 2,619 | 2,097 | 1,378
Operating segments | Reinsurance | Property excluding property catastrophe |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 2,148 | 1,645 | 1,090
Operating segments | Reinsurance | Casualty |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 1,088 | 1,005 | 855
Operating segments | Reinsurance | Property catastrophe |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 959 | 742 | 367
Operating segments | Reinsurance | Marine and aviation |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 276 | 229 | 159
Operating segments | Mortgage |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 1,231 | 1,158 | 1,160
Net premiums written | 1,112 | 1,052 | 1,133
Operating segments | Mortgage | United States |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums written | 823 | 743 | 781
Operating segments | Mortgage | Other |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums written | 289 | 309 | 352
Operating segments | Mortgage | U.S. primary mortgage insurance |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 845 | 759 | 804
Operating segments | Mortgage | U.S. credit risk transfer (CRT) and other |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 213 | 220 | 196
Operating segments | Mortgage | International mortgage insurance/reinsurance |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | $ 173 | $ 179 | $ 160

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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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