Reinsurance - Effects Of Reinsurance (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Premiums and other considerations:
Premiums and other considerations, Direct	$ 2,065.5	$ 1,618.9	$ 3,869.8	$ 3,182.2
Premiums and other considerations, Ceded	(138.9)	(126.2)	(258.6)	(240.9)
Net premiums and other considerations	1,926.6	1,492.7	3,611.2	2,941.3
Benefits, claims and settlement expenses:
Benefits, claims and settlement expenses, Direct	2,778.7	2,254.1	5,266.2	4,361.9
Benefits, claims and settlement expenses, Ceded	(700.8)	(420.0)	(1,118.6)	(753.9)
Net benefits, claims and settlement expenses	2,077.9	1,834.1	4,147.6	3,608.0
Liability for future policy benefits remeasurement (gain) loss:
Liability for future policy benefits remeasurement (gain) loss, Direct	4.4	11.6	32.4	24.3
Liability for future policy benefits remeasurement (gain) loss, Ceded	440.4	(10.9)	410.7	(29.2)
Net liability for future policy benefits remeasurement (gain) loss	$ 444.8	$ 0.7	$ 443.1	$ (4.9)

Amount, before premiums ceded to other entities, of premiums earned and premiums assumed by the entity.

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Amount for policies ceded, of remeasurement gain (loss) on liability for future policy benefit from application of revised benefit ratio as of contract issue. Includes, but is not limited to, remeasurement gain (loss) for annuitization insurance benefit, death and other insurance benefits, and traditional and limited-payment contracts.

+ References

No definition available.

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Amount before effect of policies ceded, of remeasurement gain (loss) on liability for future policy benefit from application of revised benefit ratio as of contract issue. Includes, but is not limited to, remeasurement gain (loss) for annuitization insurance benefit, death and other insurance benefits, and traditional and limited-payment contracts.

+ References

No definition available.

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Amount, before effect of policies ceded, of expense related to provision for policy benefits and costs incurred and policies assumed.

+ References

No definition available.

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Amount of earned premiums ceded to other entities.

+ References

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Amount of remeasurement gain (loss) on liability for future policy benefit from application of revised benefit ratio as of contract issue. Includes, but is not limited to, remeasurement gain (loss) for annuitization insurance benefit, death and other insurance benefits, and traditional and limited-payment contracts.

+ References

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Amount, after effects of policies assumed or ceded, of expense related to the provision for policy benefits and costs incurred.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
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Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned.

+ References

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

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
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