Reinsurance - Effects Of Reinsurance (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Premiums and other considerations:
Premiums and other considerations, Direct	$ 1,661.4	$ 1,535.6	$ 5,082.9	$ 5,405.4
Premiums and other considerations, Ceded	(128.6)	(122.7)	(402.3)	(381.3)
Net premiums and other considerations	1,532.8	1,412.9	4,680.6	5,024.1
Benefits, claims and settlement expenses:
Benefits, claims and settlement expenses, Direct	2,356.8	2,209.4	7,277.1	7,475.6
Benefits, claims and settlement expenses, Ceded	(392.7)	(431.3)	(1,253.1)	(1,549.9)
Net benefits, claims and settlement expenses	1,964.1	1,778.1	6,024.0	5,925.7
Liability for future policy benefits remeasurement (gain) loss:
Liability for future policy benefits remeasurement (gain) loss, Direct	51.8	221.5	46.1	253.9
Liability for future policy benefits remeasurement (gain) loss, Ceded	9.1	(99.0)	12.7	311.7
Net liability for future policy benefits remeasurement (gain) loss	$ 60.9	$ 122.5	$ 58.8	$ 565.6

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.

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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.

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

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