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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, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned |  | $ 12,440 | $ 9,679 | $ 8,082
Net premiums written |  | 13,468 | 11,078 | 9,017
Operating segments | Insurance |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned |  | 5,446 | 4,560 | 3,625
Net premiums written |  | 5,862 | 5,021 | 4,149
Operating segments | Insurance | United States |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums written | [1] | 3,780 | 3,342 | 2,812
Operating segments | Insurance | Europe |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums written | [1] | 1,774 | 1,405 | 1,126
Operating segments | Insurance | Other geographic location |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums written | [1] | 308 | 274 | 211
Operating segments | Insurance | Professional lines |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 1,419 | 1,314 | 943
Operating segments | Insurance | Property, energy, marine and aviation |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 1,064 | 772 | 668
Operating segments | Insurance | Programs |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 658 | 590 | 507
Operating segments | Insurance | Construction and national accounts |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 561 | 432 | 417
Operating segments | Insurance | Travel, accident and health |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 557 | 492 | 256
Operating segments | Insurance | Excess and surplus casualty |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 486 | 393 | 318
Operating segments | Insurance | Warranty and lenders solutions |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 185 | 128 | 153
Operating segments | Insurance | Other |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 516 | 439 | 363
Operating segments | Reinsurance |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned |  | 5,836 | 3,959 | 2,841
Net premiums written |  | 6,554 | 4,924 | 3,254
Operating segments | Reinsurance | United States |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums written | [1] | 1,756 | 1,247 | 829
Operating segments | Reinsurance | Bermuda |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums written | [1] | 3,288 | 2,561 | 1,555
Operating segments | Reinsurance | Europe and other |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums written | [1] | 1,510 | 1,116 | 870
Operating segments | Reinsurance | Other specialty |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 2,097 | 1,378 | 819
Operating segments | Reinsurance | Property excluding property catastrophe |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 1,645 | 1,090 | 838
Operating segments | Reinsurance | Casualty |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 1,005 | 855 | 667
Operating segments | Reinsurance | Property catastrophe |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 742 | 367 | 280
Operating segments | Reinsurance | Marine and aviation |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 229 | 159 | 153
Operating segments | Reinsurance | Other |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 118 | 110 | 84
Operating segments | Mortgage |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned |  | 1,158 | 1,160 | 1,283
Net premiums written |  | 1,052 | 1,133 | 1,261
Operating segments | Mortgage | United States |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums written |  | 743 | 781 | 913
Operating segments | Mortgage | Other geographic location |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums written |  | 309 | 352 | 348
Operating segments | Mortgage | US Primary Mortgage Insurance |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned |  | 759 | 804 | 953
Operating segments | Mortgage | US Credit Risk Transfer And Other |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned |  | 220 | 196 | 181
Operating segments | Mortgage | International Mortgage Insurance And Reinsurance |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned |  | 179 | 160 | 149
Operating segments | Other |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned |  | 0 | 0 | 333
Net premiums written |  | 0 | 0 | 353
Operating segments | Other | United States |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums written | [1] | 0 | 0 | 63
Operating segments | Other | Europe |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums written | [1] | 0 | 0 | 91
Operating segments | Other | Bermuda |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums written | [1] | 0 | 0 | 199
Operating segments | Other | Other specialty |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 0 | 0 | 118
Operating segments | Other | Property excluding property catastrophe |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 0 | 0 | 7
Operating segments | Other | Casualty |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 0 | 0 | 138
Operating segments | Other | Property catastrophe |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 0 | 0 | 15
Operating segments | Other | Marine and aviation |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 0 | 0 | 0
Operating segments | Other | Other |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | $ 0 | $ 0 | $ 55
[1] | Segment results include intersegment transactions.

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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//1943274/2147480149/944-235-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//1943274/2147479655/944-605-50-1 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//1943274/2147483589/942-220-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//1943274/2147483586/944-220-S99-1 + Details
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- Definition Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums written. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (c)(1) -SubTopic 605 -Topic 944 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479655/944-605-50-1 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b)(1) -SubTopic 605 -Topic 944 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479655/944-605-50-1 + Details
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