Market Risk Benefits - Net Asset (Liability) Balances and Changes in Valuation of the MRBs for Individual Variable Annuities (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Net asset (liability) balances and the changes in the valuation of the MRBs
Balance at beginning of period, market risk benefits	$ 20.2
Balance at end of period, market risk benefits	102.3	$ 20.2
Retirement and Income Solutions | Individual variable annuities
Net asset (liability) balances and the changes in the valuation of the MRBs
Balance at beginning of period, market risk benefits	41.5	(72.2)
Effect of changes in nonperformance risk at beginning of period	7.7	(31.7)
Adjusted balance at beginning of period, market risk benefits	49.2	(103.9)
Interest accrual and expected policyholder behavior	(20.3)	(80.9)
Benefit payments	0.3	0.4
Changes in interest rates	44.6	39.9
Changes in equity markets	54.8	155.1
Changes in equity index volatility	12.8	47.9
Actual policyholder behavior different from expected behavior	(0.4)	(4.0)
Changes in other future expected assumptions		(5.3)
Adjusted balance at end of period, market risk benefits	141.0	49.2
Effect of changes in nonperformance risk at end of period	18.4	7.7
Balance at end of period, market risk benefits	$ 122.6	$ 41.5
Weighted-average attained age of policyholders (years)	67 years 8 months 12 days	67 years 8 months 12 days
Net amount at risk	$ 54.4	$ 111.2