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Reserves for Losses and Adjustment Expenses Short-duration insurance contracts - Reconciliation of Claims Development (Details) - USD ($) $ in Millions | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
--- | --- | --- | --- | ---
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for claims and claim adjustment expenses, net of reinsurance | $ 3,629.6 | $ 3,151.6 |  |
Reinsurance recoverable on unpaid losses | 4,577.8 | 4,897.7 |  |
Unallocated claims incurred | 47.9 | 41.4 |  |
Carbon syndicate reserves | 16.7 | 5.2 |  |
Other | 0.4 | 1.6 |  |
Liability for unpaid claims and claim adjustment expense, aggregate reconciling items | (396.8) | (338.4) |  |
Losses and loss adjustment expenses | 7,810.6 | 7,710.9 | $ 7,611.8 | $ 7,165.3
Property Insurance |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for claims and claim adjustment expenses, net of reinsurance | 270.8 | 259.0 |  |
Casualty Insurance |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for claims and claim adjustment expenses, net of reinsurance | 636.5 | 457.3 |  |
Marine, Aviation and Energy Insurance |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for claims and claim adjustment expenses, net of reinsurance | 194.5 | 163.7 |  |
Financial and Professional Insurance |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for claims and claim adjustment expenses, net of reinsurance | 837.6 | 671.9 |  |
Insurance lines |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for claims and claim adjustment expenses, net of reinsurance | 1,939.4 | 1,551.9 |  |
Reinsurance recoverable on unpaid losses | 2,821.6 | 2,907.8 |  |
Property Catastrophe and Other Property Reinsurance |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for claims and claim adjustment expenses, net of reinsurance | 562.3 | 677.9 |  |
Casualty Reinsurance |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for claims and claim adjustment expenses, net of reinsurance | 733.6 | 571.6 |  |
Specialty Reinsurance |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for claims and claim adjustment expenses, net of reinsurance | 394.3 | 350.2 |  |
Reinsurance lines |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for claims and claim adjustment expenses, net of reinsurance | 1,690.2 | 1,599.7 |  |
Reinsurance recoverable on unpaid losses | 1,756.2 | 1,989.9 |  |
Insurance lines other than short-duration |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Other reinsurance balances recoverable | (489.1) | (429.3) |  |
Losses and loss adjustment expenses | $ 27.3 | $ 42.7 |  |

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- Definition Short-duration Insurance Contracts, Liability for Unpaid Claims and Claims Adjustment Expenses, Other Reconciling Item, Carbon Syndicate Reserves + References No definition available. + Details
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- Definition Short-duration Insurance Contracts, Liability for Unpaid Claims and Claims Adjustment Expense, Other Reconciling Item, Other Reinsurance Balances Recoverable + References No definition available. + Details
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- Definition The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -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 valuation allowance, recoverable under reinsurance contracts for losses reported to the ceding insurer but not yet paid and amounts expected for incurred losses and settlement expenses, which have not yet been reported to the ceding insurer. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 310 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479584/944-310-50-2 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -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 3: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 310 -Name Accounting Standards Codification -Section 45 -Paragraph 6 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479616/944-310-45-6 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -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 5: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -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 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://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 4B -SubTopic 40 -Topic 944 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-4B Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 4C -SubTopic 40 -Topic 944 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-4C + Details
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- Definition Amount of aggregate reconciling items for the reconciliation of the liability in the disclosure of claims development (in excess of) less than the liability for unpaid claims and claims adjustment expense for short-duration insurance contracts. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 4C -SubTopic 40 -Topic 944 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-4C + Details
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- Definition Amount of reconciling item for the reconciliation of the liability in the disclosure of claims development (in excess of) less than the liability for unpaid claims and claims adjustment expense for short-duration insurance contracts, classified as other. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 4C -SubTopic 40 -Topic 944 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-4C + Details
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