Derivative Financial Instruments - Fair Value Hedges (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Fair Value Hedges
Carrying amount of hedged asset - Active hedging relationships	$ 3,399.5	$ 3,510.1
Hedged Asset, Statement of Financial Position [Extensible Enumeration]	Debt Securities, Available-for-Sale, Excluding Accrued Interest	Debt Securities, Available-for-Sale, Excluding Accrued Interest
Carrying amount of hedged asset - Discontinued hedging relationships	$ 340.8	$ 304.7
Carrying amount of hedged asset - Active or discontinued hedging relationships	3,740.3	3,814.8
Cumulative amount of fair value hedging basis adjustment - Active hedging relationships	(107.3)	(87.0)
Cumulative amount of fair value hedging basis adjustment - Discontinued hedging relationships	(6.2)	(5.2)
Cumulative amount of fair value hedging basis adjustment - Active or discontinued hedging relationships	(113.5)	(92.2)
Carrying amount of hedged liability - Active hedging relationships	$ 1,298.4	$ 300.5
Hedged Liability, Statement of Financial Position [Extensible Enumeration]	Contractholder Funds.	Contractholder Funds.
Carrying amount of hedged liability - Active or discontinued hedging relationships	$ 1,298.4	$ 300.5
Cumulative amount of fair value hedging basis adjustment - Active hedging relationships	(14.5)	0.4
Cumulative amount of fair value hedging basis adjustment - Active or discontinued hedging relationships	(14.5)	0.4
Amortized cost basis of closed portfolio used in portfolio layer hedging relationship	3,092.4	3,178.5
Cumulative basis adjustments associated with portfolio layer hedging relationship	(75.9)	(62.2)
Amount of hedged item in portfolio layer hedging relationship	1,295.0	1,395.0
Gain (loss) from components excluded from assessment of fair value hedge effectiveness	$ 0.0	$ 0.0