Total Cash, Cash Equivalents, Restricted Cash and Restricted Investments - Summary of Cash and Cash Equivalents, Restricted Cash and Restricted Investments (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Dec. 31, 2022
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 806.8	$ 712.4	$ 569.0
Restricted cash and cash equivalents	212.3	251.7	261.5
Total cash, cash equivalents and restricted cash	1,019.1	964.1	$ 830.5	$ 1,407.9
Restricted investments securing reinsurance contracts and letter of credit facilities	1,225.7	1,347.0
Total cash, cash equivalents, restricted cash and restricted investments	2,244.8	2,311.1
Financial Standby Letter of Credit
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and cash equivalents	8.9	5.8
Reinsurance Contracts
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and cash equivalents	$ 203.4	$ 245.9

Cash, Cash Equivalents, Restricted Cash, Restricted Cash Equivalents And Restricted Investments

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Amount of cash and cash equivalents, and cash and cash equivalents restricted to withdrawal or usage. Excludes amount for disposal group and discontinued operations. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Amount of cash and cash equivalents restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Investments which are not defined as or included in marketable (debt, equity, or other) securities whose use is restricted in whole or in part, generally by contractual agreements or regulatory requirements. For use in an unclassified balance sheet.

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