- Reinsurance Recoverables (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Line Items]
Reinsurance recoverables, net	$ 2,820.9	$ 3,104.6
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Balance, January 1, 2020	1.1
Current period change for estimated uncollectible reinsurance	0.0
Write-offs of uncollectible reinsurance recoverables	0.0
Balance, March 31, 2020	$ 3.6
Percent rated A- or better	87.60%
Cumulative Effect, Period Of Adoption, Adjustment
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Balance, January 1, 2020	$ 2.5

Credit Risk Rating, Percent Rated A- or Better

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- Definition

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

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- Definition

Amount of the write-down or write-off of a receivable from a reinsurer.

+ 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

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Amount of expense (reversal of expense) for expected credit loss on reinsurance recoverable.

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- Definition

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://fasb.org/us-gaap/role/ref/legacyRef
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- Definition

Amount of allowance for credit loss on reinsurance recoverable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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