Fair Value Measurements - Gross Purchases, Sales, Issuances and Settlements (Details) - Recurring Fair Value Measurements - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
05 Gross purchases, sales, issuances and settlements
Purchases, net derivative assets (liabilities)	$ 1.1	$ (2.2)	$ 1.4	$ (1.3)
Sales, net derivative assets (liabilities)	0.3	1.9	1.2	2.2
Net purchases, sales, issuances and settlements, net derivative assets (liabilities)	1.4	(0.3)	2.6	0.9
Investment and universal life contracts
05 Gross purchases, sales, issuances and settlements
Issuances, liabilities	(148.2)	(155.1)	(389.4)	(469.1)
Settlements, liabilities	14.7	12.2	46.5	39.9
Net purchases, sales, issuances and settlements, liabilities	(133.5)	(142.9)	(342.9)	(429.2)
Fixed maturities \| Available-for-sale
05 Gross purchases, sales, issuances and settlements
Purchases, assets	334.0	218.8	906.4	1,056.6
Sales, assets	(18.1)	(28.9)	(310.9)	(362.2)
Settlements, assets	(220.9)	(191.9)	(693.4)	(383.9)
Net purchases, sales, issuances and settlements, assets	95.0	(2.0)	(97.9)	310.5
States and political subdivisions \| Available-for-sale
05 Gross purchases, sales, issuances and settlements
Settlements, assets	(0.4)	(0.5)	(1.4)	(1.4)
Net purchases, sales, issuances and settlements, assets	(0.4)	(0.5)	(1.4)	(1.4)
Corporate debt securities \| Available-for-sale
05 Gross purchases, sales, issuances and settlements
Purchases, assets	240.3	125.7	618.6	449.9
Sales, assets	(18.1)	(28.9)	(88.9)	(184.5)
Settlements, assets	(175.9)	(107.7)	(418.6)	(190.7)
Net purchases, sales, issuances and settlements, assets	46.3	(10.9)	111.1	74.7
Commercial mortgage-backed securities \| Available-for-sale
05 Gross purchases, sales, issuances and settlements
Settlements, assets	(0.1)	(0.1)	(0.3)	(0.3)
Net purchases, sales, issuances and settlements, assets	(0.1)	(0.1)	(0.3)	(0.3)
Other debt obligations \| Available-for-sale
05 Gross purchases, sales, issuances and settlements
Purchases, assets	93.7	93.1	287.8	606.7
Sales, assets			(222.0)	(177.7)
Settlements, assets	(44.5)	(83.6)	(273.1)	(191.5)
Net purchases, sales, issuances and settlements, assets	49.2	9.5	(207.3)	237.5
Fixed maturities, trading
05 Gross purchases, sales, issuances and settlements
Purchases, assets	95.2	68.5	237.4	231.1
Sales, assets	(36.2)	(22.4)	(206.8)	(144.0)
Settlements, assets	(11.8)	(34.2)	(24.3)	(129.2)
Net purchases, sales, issuances and settlements, assets	47.2	11.9	6.3	(42.1)
Mortgage loans
05 Gross purchases, sales, issuances and settlements
Settlements, assets				(140.6)
Net purchases, sales, issuances and settlements, assets				(140.6)
Other investments
05 Gross purchases, sales, issuances and settlements
Purchases, assets	0.7	58.7	9.5	119.6
Sales, assets			(0.7)	(1.5)
Settlements, assets	(23.1)	(39.0)	(103.1)	(110.1)
Net purchases, sales, issuances and settlements, assets	(22.4)	19.7	(94.3)	8.0
Separate account assets
05 Gross purchases, sales, issuances and settlements
Purchases, assets	1.5		1.5
Sales, assets		(42.6)		(42.6)
Settlements, assets		$ 42.6		$ 42.6
Net purchases, sales, issuances and settlements, assets	$ 1.5		$ 1.5

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- Definition

Amount of net issuances of financial instruments measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

No definition available.

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Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of purchases, (sales), issuances and (settlements) of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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