Future Policy Benefits and Claims - Corporate (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Present value of expected future policy benefit payments
Future policy benefits	$ 35,747.6	$ 36,514.6
Corporate | Long-term care insurance
Present value of expected net premiums
Balance at beginning of period, Present value of expected net premiums	42.8	64.6
Effect of changes in discount rate assumptions at beginning of period	(3.0)	(3.6)
Balance at beginning of period at original discount rate	39.8	61.0
Effect of changes in cash flow assumptions		(13.3)
Effect of actual variances from expected experience	(1.7)	(5.7)
Adjusted beginning of period balance at original discount rate	38.1	42.0
Interest accrual	0.6	2.9
Net premiums collected	(1.6)	(5.1)
Balance at end of period at original discount rate	37.1	39.8
Effect of changes in discount rate assumptions at end of period	2.1	3.0
Balance at end of period, Present value of expected net premiums	39.2	42.8
Present value of expected future policy benefit payments
Balance at beginning of period, Present value of expected future policy benefit payments	209.5	248.1
Effect of changes in discount rate assumptions at beginning of period	(20.0)	(18.9)
Balance at beginning of period at original discount rate	189.5	229.2
Effect of changes in cash flow assumptions		(40.5)
Effect of actual variances from expected experience	0.5	2.5
Adjusted beginning of period balance at original discount rate	190.0	191.2
Interest accrual	2.8	12.5
Benefit payments	(3.6)	(14.2)
Balance at end of period at original discount rate	189.2	189.5
Effect of changes in discount rate assumptions at end of period	14.5	20.0
Balance at end of period, Present value of expected future policy benefit payments	203.7	209.5
Future policy benefits	164.5	166.7
Reinsurance Impact	$ (164.5)	$ (166.7)
Weighted-average duration for future policy benefits (in years)	10 years 1 month 6 days	10 years 4 months 24 days
Increase (decrease) to LFPB due to effect of changes in discount rate assumptions	$ (4.6)	$ 1.7