Reinsurance (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Effects Of Reinsurance [Line Items]
Decrease in reinsurance receivables	$ 91.6	$ 57.2	$ 32.9
Property and casualty premiums earned, Ceded	629.4	612.9	388.8
Property and casualty losses and LAE, Ceded	397.3	361.6	290.6
Reinsurance recoverable on paid and unpaid losses and unearned premiums	2,056.1	1,964.5
MCCA [Member]
Effects Of Reinsurance [Line Items]
Estimated surplus to policyholders			3,000.0
Refund amount			183.2
Decrease in reinsurance receivables			(122.9)
Property and casualty premiums earned, Ceded	32.9	34.6
Property and casualty losses and LAE, Ceded	72.8	32.6	68.0
Reinsurance recoverable on paid and unpaid losses and unearned premiums	$ 911.7	$ 883.0
Decrease on direct premiums earned and premiums ceded			143.7
MCCA [Member] \| Reinsurance Receivable [Member] \| Reinsurer Concentration Risk [Member]
Effects Of Reinsurance [Line Items]
Percentage of reinsurance receivable represented by segment	44.30%
MCCA [Member] \| Personal Automobile Policyholders [Member]
Effects Of Reinsurance [Line Items]
Refund amount			179.1
MCCA [Member] \| Commercial Automobiles Policyholders [Member]
Effects Of Reinsurance [Line Items]
Refund amount			$ 4.1

Estimated surplus to policyholders.

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Increase (decrease) on direct premiums earned and premiums ceded.

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No definition available.

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Refund amount.

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- Definition

Amount of earned premiums ceded to other entities.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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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

No definition available.

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The increase (decrease) during the reporting period in the amount of benefits the ceding insurer expects to recover on insurance policies ceded to other insurance entities as of the balance sheet date for all guaranteed benefit types.

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

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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.

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