Long-Duration Insurance Contracts - Deferred Policy Acquisition Costs, Rollforward (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period	$ 336.3
Amortization expense	(27.0)	$ (23.7)
Balance, end of period	337.3
Whole Life Insurance
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period	22.3	20.9
Capitalizations	0.7	0.6
Amortization expense	(0.3)	(0.3)
Experience adjustment	(0.1)	0.0
Balance, end of period	22.6	21.2
Term Life Insurance
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period	32.6	30.0
Capitalizations	1.2	1.3
Amortization expense	(0.9)	(0.6)
Experience adjustment	0.0	0.0
Balance, end of period	32.9	30.7
Experience Life Insurance
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period	5.7	5.8
Capitalizations	0.1	0.0
Amortization expense	(0.1)	0.0
Experience adjustment	0.0	0.0
Balance, end of period	5.7	5.8
Limited Pay Whole Life Insurance
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period	7.4	6.7
Capitalizations	0.2	0.2
Amortization expense	(0.1)	(0.1)
Experience adjustment	0.0	0.0
Balance, end of period	7.5	6.8
Indexed Universal Life Insurance
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period	16.8	15.4
Capitalizations	0.7	0.5
Amortization expense	(0.3)	(0.2)
Experience adjustment	(0.1)	0.0
Balance, end of period	17.1	15.7
Supplemental health
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period	8.2	6.2
Capitalizations	0.8	0.6
Amortization expense	(0.2)	(0.1)
Experience adjustment	(0.1)	0.0
Balance, end of period	8.7	6.7
Total Annuities
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period	214.0	221.1
Capitalizations	3.9	4.0
Amortization expense	(3.7)	(3.7)
Experience adjustment	(1.3)	(1.7)
Balance, end of period	$ 212.9	$ 219.7