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, 2025	Dec. 31, 2024	Sep. 30, 2024	Dec. 31, 2023
Restricted Cash and Cash Equivalent Item [Line Items]
Cash and cash equivalents	$ 892.5	$ 743.0	$ 806.8
Restricted cash and cash equivalents	196.5	203.6	212.3
Total cash, cash equivalents and restricted cash	1,089.0	946.6	$ 1,019.1	$ 964.1
Restricted investments securing reinsurance contracts and letter of credit facilities	1,090.1	1,328.7
Total cash, cash equivalents, restricted cash and restricted investments	2,179.1	2,275.3
Financial Standby Letter of Credit
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash and cash equivalents	17.3	51.6
Reinsurance Contracts
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash and cash equivalents	$ 179.2	$ 152.0

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

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Amount of cash and cash equivalent. Cash includes, but is not limited to, currency on hand, demand deposit with financial institution, and account with general characteristic of demand deposit. Cash equivalent includes, but is not limited to, short-term, highly liquid investment that is both readily convertible to known amount of cash and so near maturity that it presents insignificant risk of change in value because of change in interest rate.

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Amount of cash and cash equivalent, and cash and cash equivalent restricted to withdrawal or usage; attributable to continuing operation. Cash includes, but is not limited to, currency on hand, demand deposit with financial institution, and account with general characteristic of demand deposit. Cash equivalent includes, but is not limited to, short-term, highly liquid investment that is both readily convertible to known amount of cash and so near maturity that it presents insignificant risk of change in value because of change in interest rate.

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Amount of cash and cash equivalent restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposit with financial institution, and account with general characteristic of demand deposit. Cash equivalent includes, but is not limited to, short-term, highly liquid investment that is both readily convertible to known amount of cash and so near maturity that it presents insignificant risk of change in value because of change in interest rate.

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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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