Reserve for Known and Incurred but Not Reported Claims (Activity in Reserve for Known and Incurred but Not Reported Claims) (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balance at beginning of year	$ 1,325.3	$ 1,283.8	$ 1,178.0
Provision related to current year	354.6	468.3	569.3
Provision related to prior years	(18.3)	18.0	19.4
Total Provision	336.3	486.3	588.7
Payments, net of recoveries, related to: Current year	199.6	225.8	292.5
Payments, net of recoveries, related to: Prior years	182.2	208.5	189.8
Total Payments, net of recoveries	381.8	434.3	482.3
Other	2.6	(10.5)	(0.6)
Balance at end of year	$ 1,282.4	$ 1,325.3	$ 1,283.8

Liability for unpaid claims and claims adjustment expense incurred but not reported IBNR claims amount, other.

No definition available.

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- Definition

The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date.

+ References

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- Definition

Amount, after effects of reinsurance, of payments to settle claims incurred in the current period and related claims settlement costs.

+ References

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- Definition

Amount, after effects of reinsurance, of payments to settle claims incurred in prior periods and related claims settlement costs.

+ References

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- Definition

Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred and costs incurred in the claim settlement process.

+ References

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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- Definition

Amount, after effects of reinsurance, of payments to settle insured claims and pay costs incurred in the claims settlement process.

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- Definition

Amount, after effects of reinsurance, of expense for claims incurred in the current reporting period and related claims settlement costs.

+ References

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- Definition

Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs.

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