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Investment Information - Net realized gains and losses (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Available for sale securities: |  |  |  |
Available for sale securities, gross gains on investment sales |  | $ 259 | $ 116 | $ 81
Available for sale securities, gross losses on investment sales |  | (354) | (547) | (317)
Equity securities, at fair value : |  |  |  |
Net realized gains (losses) on securities sold |  | 62 | 61 | 75
Net unrealized gains (losses) on equity securities still held at reporting date |  | 108 | 88 | (267)
Allowance for credit losses: |  |  |  |
Allowance for credit losses - investment related |  | 0 | 3 | (44)
Allowance for credit losses - underwriting related |  | 5 | (1) | (13)
Derivative instruments |  | 8 | 59 | (75)
Other | [1] | 251 | 10 | (4)
Net realized gains (losses) |  | $ 197 | $ (165) | $ (663)
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] |  | Net realized gains (losses) | Net realized gains (losses) | Net realized gains (losses)
Fixed maturities |  |  |  |
Available for sale securities: |  |  |  |
Change in fair value of assets and liabilities accounted for using the fair value option |  | $ 3 | $ 18 | $ (71)
Other investments |  |  |  |
Available for sale securities: |  |  |  |
Change in fair value of assets and liabilities accounted for using the fair value option |  | (144) | 27 | (21)
Equity securities |  |  |  |
Available for sale securities: |  |  |  |
Change in fair value of assets and liabilities accounted for using the fair value option |  | (1) | 1 | (4)
Short-term investments |  |  |  |
Available for sale securities: |  |  |  |
Change in fair value of assets and liabilities accounted for using the fair value option |  | $ 0 | $ 0 | $ (3)
[1] | Amounts in the 2024 period include benefits from the sale of Castel Underwriting Agencies Limited and the acquisition of RMIC.

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- Definition Includes realized gains and losses on debt securities accounted for as available for sale, realized and unrealized changes in the fair value of equity securities and assets and liabilities accounted for using the fair value option, realized and unrealized gains and losses on derivative instruments and changes in the allowance for credit losses on financial assets, and all other. + References No definition available. + Details
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- Definition Amount of increase (decrease) in allowance for credit loss of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 9 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-9 + Details
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- Definition Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 9 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-9 + Details
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- Definition Amount of realized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 9 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-9 + Details
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