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Segment Information (Net Premiums Earned For Segment By Product) (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | $ 33,117 | $ 31,290 | $ 30,064
North America Commercial P&C Insurance |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 13,964 | 12,922 | 12,402
North America Commercial P&C Insurance | Property and other short-tail [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 2,423 | 1,987 | 1,861
North America Commercial P&C Insurance | Casualty and all other [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 10,812 | 10,136 | 9,773
North America Commercial P&C Insurance | Accident and Health [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 729 | 799 | 768
North American Personal P&C [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 4,866 | 4,694 | 4,593
North American Personal P&C [Member] | Automobiles [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 822 | 829 | 803
North American Personal P&C [Member] | Personal homeowner [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 3,327 | 3,183 | 3,127
North American Personal P&C [Member] | Insurance, Other [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 717 | 682 | 663
North America Agricultural Insurance |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 1,822 | 1,795 | 1,569
Overseas General Insurance |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 9,285 | 8,882 | 8,612
Overseas General Insurance | Property and other short-tail [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 2,468 | 2,244 | 2,134
Overseas General Insurance | Casualty and all other [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 2,738 | 2,494 | 2,429
Overseas General Insurance | Accident and Health [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 2,098 | 2,248 | 2,265
Overseas General Insurance | Personal lines [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 1,981 | 1,896 | 1,784
Segment Global Reinsurance [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 698 | 654 | 670
Segment Global Reinsurance [Member] | Property and other short-tail [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 104 | 131 | 123
Segment Global Reinsurance [Member] | Casualty and all other [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 421 | 381 | 377
Segment Global Reinsurance [Member] | Property catastrophe [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 173 | 142 | 170
Life Insurance |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 2,482 | 2,343 | 2,218
Life Insurance | Accident and Health [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 1,165 | 1,242 | 1,196
Life Insurance | Life Insurance Product Line [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | $ 1,317 | $ 1,101 | $ 1,022

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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 -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(1)) -URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17(Column E)) -URI http://asc.fasb.org/extlink&oid=120401096&loc=d3e574992-122915 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04(13)(c)) -URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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