Derivative Financial Instruments Fair Value Hedges Carrying Amount of Hedged Assets and Liabilities and Cumulative Basis Adjustments (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Derivative Instruments and Hedging Activities Disclosures
Hedged Asset, Statement of Financial Position [Extensible Enumeration]	Fixed Maturity Securities - at fair value (amortized cost of $38,269.9; $38,410.6; allowance for credit losses of $2.8; $2.2)	Fixed Maturity Securities - at fair value (amortized cost of $38,269.9; $38,410.6; allowance for credit losses of $2.8; $2.2)
Receive Fixed/Pay Fixed \| Designated as Hedging Instrument \| Fair Value Hedging
Derivative Instruments and Hedging Activities Disclosures
Hedged Asset, Fair Value Hedge	$ 551.0	$ 529.2
Receive Fixed/Pay Fixed \| Designated as Hedging Instrument \| Fair Value Hedging \| Fixed Maturity Securities
Derivative Instruments and Hedging Activities Disclosures
Hedged Asset, Fair Value Hedge, Cumulative Increase (Decrease)	(46.3)	(6.1)
Hedged asset, amortized cost	$ 648.4	$ 636.9

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+ References

No definition available.

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- Definition

Amount of asset hedged in fair value hedging relationship.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 181
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480401/815-10-55-181

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
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-Name Accounting Standards Codification
-Section 50
-Paragraph 4EE
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-Publisher FASB
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- Definition

Amount of cumulative increase (decrease) in fair value of hedged asset in fair value hedge, attributable to hedged risk.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 181
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480401/815-10-55-181

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
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-Name Accounting Standards Codification
-Section 50
-Paragraph 4EE
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4EE

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- Definition

Indicates line item in statement of financial position that includes hedged asset.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4EE
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4EE

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-Paragraph 181
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