Derivative Financial Instruments - Fair Value Hedges (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Fair Value Hedges
Carrying amount of hedged asset - Active hedging relationships	$ 3,400.3		$ 3,400.3		$ 3,498.6
Hedged Asset, Statement of Financial Position [Extensible Enumeration]	Debt Securities, Available-for-Sale, Excluding Accrued Interest		Debt Securities, Available-for-Sale, Excluding Accrued Interest		Debt Securities, Available-for-Sale, Excluding Accrued Interest
Carrying amount of hedged asset - Discontinued hedging relationships	$ 67.8		$ 67.8		$ 48.8
Carrying amount of hedged asset - Active or discontinued hedging relationships	3,468.1		3,468.1		3,547.4
Cumulative amount of fair value hedging basis adjustment - Active hedging relationships	(150.7)		(150.7)		(153.4)
Cumulative amount of fair value hedging basis adjustment - Discontinued hedging relationships	0.3		0.3		1.3
Cumulative amount of fair value hedging basis adjustment - Active or discontinued hedging relationships	(150.4)		(150.4)		$ (152.1)
Carrying amount of hedged liability - Active hedging relationships	$ 294.8		$ 294.8
Hedged Liability, Statement of Financial Position [Extensible Enumeration]	Contractholder Funds.		Contractholder Funds.		Contractholder Funds.
Carrying amount of hedged liability - Active or discontinued hedging relationships	$ 294.8		$ 294.8
Cumulative amount of fair value hedging basis adjustment - Active hedging relationships	(9.4)		(9.4)
Cumulative amount of fair value hedging basis adjustment - Active or discontinued hedging relationships	(9.4)		(9.4)
Amortized cost basis of closed portfolio used in portfolio layer hedging relationship	3,177.4		3,177.4		$ 3,256.9
Cumulative basis adjustments associated with portfolio layer hedging relationship	(109.0)		(109.0)		(102.4)
Amount of hedged item in portfolio layer hedging relationship	1,210.0		1,210.0		$ 1,110.0
Gain (loss) from components excluded from assessment of fair value hedge effectiveness	$ (0.2)	$ 0.0	$ (1.0)	$ 0.0

Amount of asset previously hedged in a fair value hedging relationship that has been discontinued.

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Amount of asset hedged in fair value hedging relationship or previously hedged in a fair value hedging relationship that has been discontinued.

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Amount of cumulative increase (decrease) in fair value of hedged asset in fair value hedge, attributable to hedged risk, including amount remaining after discontinued hedge.

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Amount of liability hedged in fair value hedging relationship or previously hedged in a fair value hedging relationship that has been discontinued.

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Amortized cost of closed portfolio of financial asset and beneficial interest secured by financial instrument, used to designate hedging relationship in which hedged item is expected to be outstanding for designated hedge period.

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Net gain (loss) relating to components of the gain (loss) on the fair value hedging instrument excluded from the assessment of fair value hedge effectiveness. Recognized in earnings.

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

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Amount of asset hedged in fair value hedging relationship.

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Amount of asset hedged in portfolio layer hedging relationship, expected to be remaining at end of hedging relationship.

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

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

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

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