Reinsurance - Additional Information (Detail) - USD ($) $ in Millions	2 Months Ended	11 Months Ended	12 Months Ended
Dec. 31, 2023	Nov. 15, 2023	Dec. 31, 2022	Dec. 31, 2021
Ceded Credit Risk [Line Items]
Reinsurance recoverables, allowance for doubtful accounts	$ 0.0	$ 4.7	$ 4.7	$ 3.8
Amount of collateral under reinsurance agreement	1,212.2		1,299.3
Reinsurance recoverables	2,959.3		3,029.1
Paid loss recoverables in insurance recoverables	161.9		190.6
Loss & LAE	94.2	1,043.2	1,166.9	1,314.6
Net of amounts ceded to reinsurers	79.3	$ 607.3	958.8	$ 829.6
Reinsurance Recoverable for Paid and Unpaid Claims and Claims Adjustments \| Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables	945.0		1,244.3
Reinsurance Recoverable for Paid and Unpaid Claims and Claims Adjustments \| Ceded Credit Risk, Unsecured \| Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables	$ 465.1		$ 564.0

Collateral pledge as security for payments to be received under reinsurance contracts.

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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 944
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Amount of reduction of provision for policy benefits and costs incurred for policies ceded.

+ References

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- Definition

Amount of allowance for credit loss on reinsurance recoverable.

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Amount, after valuation allowance, recoverable under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, policy benefits and policy reserves. Excludes premiums paid under reinsurance contracts.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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Amount, after valuation allowance, recoverable under reinsurance contracts for losses reported to and paid by the ceding insurer.

+ References

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Amount of insurance benefit, claim, loss and settlement expenses incurred, as disclosed in supplementary insurance information.

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