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Investment Information - Net realized gains and losses (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2025 | Dec. 31, 2024 | Dec. 31, 2023
Available for sale securities: |  |  |  |
Available for sale securities, gross gains on investment sales |  | $ 296 | $ 259 | $ 116
Available for sale securities, gross losses on investment sales |  | (271) | (354) | (547)
Equity securities, at fair value : |  |  |  |
Net realized gains (losses) on securities sold |  | 84 | 62 | 61
Net unrealized gains (losses) on equity securities still held at reporting date |  | 130 | 108 | 88
Allowance for credit losses: |  |  |  |
Allowance for credit losses - investment related |  | (6) | 0 | 3
Allowance for credit losses - underwriting related |  | 3 | 5 | (1)
Derivative instruments |  | 327 | 8 | 59
Other | [1] | (169) | 251 | 10
Net realized gains (losses) |  | 464 | 197 | (165)
Fixed maturities |  |  |  |
Available for sale securities: |  |  |  |
Change in fair value of assets and liabilities accounted for using the fair value option |  | 29 | 3 | 18
Other investments |  |  |  |
Available for sale securities: |  |  |  |
Change in fair value of assets and liabilities accounted for using the fair value option |  | 38 | (144) | 27
Equity securities |  |  |  |
Available for sale securities: |  |  |  |
Change in fair value of assets and liabilities accounted for using the fair value option |  | 0 | (1) | 1
Short-term investments |  |  |  |
Available for sale securities: |  |  |  |
Change in fair value of assets and liabilities accounted for using the fair value option |  | $ 3 | $ 0 | $ 0
[1] | Amounts in the 2025 periods primarily include losses related to the sale of certain alternative investments accounted for under the equity method, while amounts in the 2024 period include benefits from the sale of Castel Underwriting Agencies Limited and the acquisition of RMIC Companies, Inc

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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 All other realized and unrealized gains and losses. + 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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- Definition Amount of realized gain (loss) from sale of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 220 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(3)(d)(1)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477250/944-220-S99-1 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 321 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479536/321-10-50-4 + Details
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