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Reinsurance (Reinsurance Recoverable by Category and Listing of Largest Reinsurers) (Details) - USD ($) $ in Millions | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
--- | --- | --- | ---
Ceded Credit Risk [Line Items] |  |  |
Gross Reinsurance Recoverable on Losses and Loss Expenses | $ 20,319 |  |
Valuation allowance for Uncollectible Reinsurance | $ 367 | $ 351 | $ 329
% of Gross Reinsurance Recoverable | 1.80% |  |
Largest reinsurers |  |  |
Ceded Credit Risk [Line Items] |  |  |
Gross Reinsurance Recoverable on Losses and Loss Expenses | $ 10,993 |  |
Valuation allowance for Uncollectible Reinsurance | $ 117 |  |
% of Gross Reinsurance Recoverable | 1.10% |  |
Other reinsurers rated A- or better |  |  |
Ceded Credit Risk [Line Items] |  |  |
Gross Reinsurance Recoverable on Losses and Loss Expenses | $ 4,898 |  |
Valuation allowance for Uncollectible Reinsurance | $ 57 |  |
% of Gross Reinsurance Recoverable | 1.20% |  |
Other reinsurers rated lower than A- or not rated |  |  |
Ceded Credit Risk [Line Items] |  |  |
Gross Reinsurance Recoverable on Losses and Loss Expenses | $ 455 |  |
Valuation allowance for Uncollectible Reinsurance | $ 56 |  |
% of Gross Reinsurance Recoverable | 12.30% |  |
Pools |  |  |
Ceded Credit Risk [Line Items] |  |  |
Gross Reinsurance Recoverable on Losses and Loss Expenses | $ 441 |  |
Valuation allowance for Uncollectible Reinsurance | $ 14 |  |
% of Gross Reinsurance Recoverable | 3.20% |  |
Structured settlements |  |  |
Ceded Credit Risk [Line Items] |  |  |
Gross Reinsurance Recoverable on Losses and Loss Expenses | $ 493 |  |
Valuation allowance for Uncollectible Reinsurance | $ 11 |  |
% of Gross Reinsurance Recoverable | 2.20% |  |
Captives |  |  |
Ceded Credit Risk [Line Items] |  |  |
Gross Reinsurance Recoverable on Losses and Loss Expenses | $ 2,653 |  |
Valuation allowance for Uncollectible Reinsurance | $ 16 |  |
% of Gross Reinsurance Recoverable | 0.60% |  |
Other |  |  |
Ceded Credit Risk [Line Items] |  |  |
Gross Reinsurance Recoverable on Losses and Loss Expenses | $ 386 |  |
Valuation allowance for Uncollectible Reinsurance | $ 96 |  |
% of Gross Reinsurance Recoverable | 24.90% |  |

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- Definition Amount of allowance for credit loss on reinsurance recoverable. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479838/944-20-50-5 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 310 -Name Accounting Standards Codification -Section 45 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479616/944-310-45-5 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479344/326-20-45-1 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 13 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479319/326-20-50-13 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 13 -Subparagraph (f) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479319/326-20-50-13 + Details
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- Definition Amount before valuation allowance recoverable under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479838/944-20-50-5 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(6)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479440/944-210-S99-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 310 -Name Accounting Standards Codification -Section 45 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479616/944-310-45-5 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 14 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479319/326-20-50-14 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section 50 -Paragraph 7A -SubTopic 10 -Topic 310 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481962/310-10-50-7A + Details
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