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Business segment data - Insurance premiums by type (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Property/Casualty [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Premiums written - Direct | $ 61,990 | $ 56,700 | $ 53,829
Premiums written - Assumed | 20,751 | 15,143 | 12,461
Premiums written - Ceded | (2,402) | (1,155) | (1,015)
Premiums written | 80,339 | 70,688 | 65,275
Premiums earned - Direct | 60,437 | 55,879 | 52,139
Premiums earned - Assumed | 20,442 | 14,184 | 12,072
Premiums earned - Ceded | (2,548) | (1,293) | (1,054)
Premiums earned | 78,331 | 68,770 | 63,157
Life/Health [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Premiums written - Direct |  | 582 | 646
Premiums written - Assumed | 5,126 | 5,222 | 5,670
Premiums written - Ceded | (33) | (37) | (40)
Premiums written | 5,093 | 5,767 | 6,276
Premiums earned - Direct |  | 582 | 646
Premiums earned - Assumed | 5,105 | 5,263 | 5,698
Premiums earned - Ceded | (33) | (39) | (41)
Premiums earned | $ 5,072 | $ 5,806 | $ 6,303

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- Definition Amount of earned premiums assumed from other entities. + 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 D)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480149/944-235-S99-2 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 220 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(1)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483586/944-220-S99-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (c)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479655/944-605-50-1 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479655/944-605-50-1 + Details
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- Definition Amount of written premiums assumed from other entities. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479655/944-605-50-1 + Details
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- Definition Amount, before 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 B)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480149/944-235-S99-2 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 220 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(1)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483586/944-220-S99-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479655/944-605-50-1 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (c)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479655/944-605-50-1 + Details
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