Third-Party Reinsurance - Reinsurance Recoverables (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Segment Reporting Information [Line Items]
Reinsurance recoverables on unpaid losses	$ 352.4	$ 505.0
Reinsurance recoverables on paid losses	26.3	31.1
Ceded unearned premiums	159.7	59.2
Reinsurance recoverables	538.4	595.3
P&C Insurance and Reinsurance (Ark/WM Outrigger) \| Ark
Segment Reporting Information [Line Items]
Reinsurance recoverables on unpaid losses	(7.9)
Ceded unearned premiums	$ (33.0)
TPC Providers
Segment Reporting Information [Line Items]
Reinsurance recoverables on unpaid losses		$ 145.4

The unexpired portion of premiums ceded on policies in force as of the balance sheet date.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Amount, after valuation allowance, recoverable under reinsurance contracts for losses reported to the ceding insurer but not yet paid and amounts expected for incurred losses and settlement expenses, which have not yet been reported to the ceding insurer.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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Amount, after valuation allowance, recoverable under reinsurance contracts including premium paid under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves.

+ 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.

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Reinsurance Recoverables on Unpaid Losses, Ceded Unearned Premiums

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