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Claim and Claim Adjustment Expense Reserves (Specialty - Surety - Net Cumulative Paid Claim and Claim Adjustment Expenses) (Details) - USD ($) $ in Millions | Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
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Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Total net liability for unpaid claim and claim adjustment expenses | $ 19,263 | $ 18,163 | $ 16,929 | $ 16,300 |  |  |  |  |  |
Specialty |  |  |  |  |  |  |  |  |  |
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Total net liability for unpaid claim and claim adjustment expenses | 5,979 | 5,916 |  |  |  |  |  |  |  |
Surety | Specialty |  |  |  |  |  |  |  |  |  |
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative paid claims and allocated claim adjustment expense, net | 446 |  |  |  |  |  |  |  |  |
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented | 460 |  |  |  |  |  |  |  |  |
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2015 | 14 |  |  |  |  |  |  |  |  |
Liability for unallocated claim adjustment expenses for accident years presented | 19 |  |  |  |  |  |  |  |  |
Total net liability for unpaid claim and claim adjustment expenses | 493 |  |  |  |  |  |  |  |  |
Surety | Specialty | 2015 |  |  |  |  |  |  |  |  |  |
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative paid claims and allocated claim adjustment expense, net | 43 | 43 | 42 | 42 | $ 44 | $ 42 | $ 40 | $ 38 | $ 26 | $ 7
Surety | Specialty | 2016 |  |  |  |  |  |  |  |  |  |
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative paid claims and allocated claim adjustment expense, net | 40 | 40 | 41 | 43 | 43 | 45 | 45 | 37 | $ 5 |
Surety | Specialty | 2017 |  |  |  |  |  |  |  |  |  |
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative paid claims and allocated claim adjustment expense, net | 63 | 62 | 62 | 49 | 46 | 41 | 37 | $ 23 |  |
Surety | Specialty | 2018 |  |  |  |  |  |  |  |  |  |
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative paid claims and allocated claim adjustment expense, net | 41 | 41 | 40 | 39 | 34 | 25 | $ 5 |  |  |
Surety | Specialty | 2019 |  |  |  |  |  |  |  |  |  |
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative paid claims and allocated claim adjustment expense, net | 74 | 70 | 59 | 44 | 34 | $ 12 |  |  |  |
Surety | Specialty | 2020 |  |  |  |  |  |  |  |  |  |
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative paid claims and allocated claim adjustment expense, net | 44 | 33 | 28 | 20 | $ 4 |  |  |  |  |
Surety | Specialty | 2021 |  |  |  |  |  |  |  |  |  |
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative paid claims and allocated claim adjustment expense, net | 42 | 35 | 20 | $ 5 |  |  |  |  |  |
Surety | Specialty | 2022 |  |  |  |  |  |  |  |  |  |
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative paid claims and allocated claim adjustment expense, net | 52 | 35 | $ 12 |  |  |  |  |  |  |
Surety | Specialty | 2023 |  |  |  |  |  |  |  |  |  |
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative paid claims and allocated claim adjustment expense, net | 27 | $ 8 |  |  |  |  |  |  |  |
Surety | Specialty | 2024 |  |  |  |  |  |  |  |  |  |
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative paid claims and allocated claim adjustment expense, net | $ 20 |  |  |  |  |  |  |  |  |

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- Definition Liability as of the balance sheet date for amounts representing estimated cost of settling unpaid claims under the terms of the underlying insurance policies, less estimated reinsurance recoveries on such claims. This includes an estimate for claims which have been incurred but not reported. Claim adjustment expenses represent the costs estimated to be incurred in the settlement of unpaid claims. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(13)(a)(1)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (cc) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-3 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-3 + Details
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- Definition Amount, after reinsurance, of paid claims and allocated claim adjustment expense used in claims development for short-duration insurance contracts. Excludes unallocated claim adjustment expense. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 9E -Publisher FASB -URI https://asc.fasb.org/1943274/2147480046/944-40-55-9E Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 4B -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-4B + Details
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- Definition Undiscounted amount, after reinsurance, of the liability for unpaid claims and allocated claim adjustment expense for short-duration insurance contracts. Excludes unallocated claim adjustment expense. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 9E -Publisher FASB -URI https://asc.fasb.org/1943274/2147480046/944-40-55-9E Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 4C -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-4C Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 4B -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-4B + Details
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- Definition Undiscounted amount, after reinsurance, of the liability for unpaid claims and allocated claim adjustment expense for short-duration insurance contracts for accident years not separately presented in claim development information. Excludes unallocated claim adjustment expense. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 9E -Publisher FASB -URI https://asc.fasb.org/1943274/2147480046/944-40-55-9E Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 4B -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-4B + Details
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- Definition Amount of liability for unallocated claim adjustment expense for short-duration insurance contracts. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 9E -Publisher FASB -URI https://asc.fasb.org/1943274/2147480046/944-40-55-9E Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 4C -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-4C + Details
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