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Segment Reporting - Summary of the Schedule of the Company's gross written premiums based on the location of the insured risk (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2024 | Sep. 30, 2023 | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Gross written premiums | $ 1,116.8 | $ 982.7 | $ 3,598.6 | $ 3,107.9 | $ 3,967.6 | $ 4,338.7 | $ 3,938.4
Australia/Asia |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Gross written premiums |  |  |  |  | 177.8 | 257.5 | 275.8
Europe |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Gross written premiums |  |  |  |  | 179.4 | 194.5 | 140.6
United Kingdom |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Gross written premiums |  |  |  |  | 532.5 | 485.8 | 393.2
United States & Canada |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Gross written premiums |  |  |  |  | 2,472.0 | 2,715.7 | 2,301.8
Worldwide excluding United States |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Gross written premiums |  |  |  |  | 28.8 | 24.2 | 31.5
Worldwide including United States |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Gross written premiums |  |  |  |  | 417.2 | 541.7 | 592.2
Other (4) |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Gross written premiums |  |  |  |  | $ 159.9 | $ 119.3 | $ 203.3

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- Definition Amount of direct and assumed premiums written. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 1 -SubTopic 605 -Topic 944 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479655/944-605-50-1 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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