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Derivative Contracts - Additional Information (Details) - USD ($) $ in Millions |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Oct. 01, 2022 | Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2024 | Sep. 30, 2023 | Dec. 31, 2023 | Dec. 31, 2022
Reinsurance Contracts [Axis]: Funds withheld credited with interest for periods after October 1, 2022 |  |  |  |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |  |  |  |
Reinsurance Retention Policy, Retrocession Premium, Interest Rate | 1.75% |  |  |  |  |  |
Level 3 | Liability | Fair Value, Recurring [Member] |  |  |  |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |  |  |  |
Liability, increase (decrease) in fv included in net income |  | $ 5.1 | $ 0.0 | $ 11.9 | $ 8.3 | $ 15.2 | $ (14.5)
Foreign exchange contracts | Not Designated as Hedging Instrument |  |  |  |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |  |  |  |
Purchase of US and foreign exchange |  | 1,585.0 |  | 1,585.0 |  | 1,802.9 | 1,675.3
Gain (loss) to net income from derivative instruments |  | 32.7 | (35.6) | 10.3 | (24.3) | 10.9 | (66.0)
Foreign exchange contracts | Net change from current period hedged transactions | Designated as Hedging Instrument |  |  |  |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |  |  |  |
Gain (loss) on foreign currency fair value hedge derivatives |  | 2.1 | (6.4) | 1.9 | (10.7) | (14.0) | 15.4
Foreign exchange contracts | General, administrative and corporate expenses | Designated as Hedging Instrument |  |  |  |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |  |  |  |
Gain (loss) on foreign currency fair value hedge derivatives |  | (0.3) | (2.4) | (0.3) | (6.1) | (8.1) | 5.9
Loss Portfolio Transfer | Level 3 | Liability | Fair Value, Recurring [Member] |  |  |  |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |  |  |  |
Liability, increase (decrease) in fv included in net income |  | $ 5.1 | $ 0.0 | $ 11.9 | $ 8.3 | $ 15.2 | $ (14.5)

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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 Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4E -Publisher FASB -URI https://asc.fasb.org//1943274/2147480434/815-10-50-4E + Details
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