Future Policy Benefits and Claims - Benefits and Protection (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Sep. 30, 2023	Dec. 31, 2023
Present value of expected future policy benefit payments
Future policy benefits	$ 35,747.6		$ 36,514.6
Benefits and Protection
Present value of expected future policy benefit payments
Future policy benefits	2,947.1		2,984.3
Benefits and Protection | Life Insurance | Term life
Present value of expected net premiums
Balance at beginning of period, Present value of expected net premiums	3,793.7		3,423.2
Effect of changes in discount rate assumptions at beginning of period	100.1		196.0
Balance at beginning of period at original discount rate	3,893.8		3,619.2
Effect of changes in cash flow assumptions			143.5
Effect of actual variances from expected experience	17.7		103.3
Adjusted beginning of period balance at original discount rate	3,911.5		3,866.0
Interest accrual	44.8		171.9
Net premiums collected	(93.8)		(359.8)
Issuances	58.9		215.7
Balance at end of period at original discount rate	3,921.4		3,893.8
Effect of changes in discount rate assumptions at end of period	(185.3)		(100.1)
Balance at end of period, Present value of expected net premiums	3,736.1		3,793.7
Present value of expected future policy benefit payments
Balance at beginning of period, Present value of expected future policy benefit payments	4,879.6		4,332.2
Effect of changes in discount rate assumptions at beginning of period	124.5		251.6
Balance at beginning of period at original discount rate	5,004.1		4,583.8
Effect of changes in cash flow assumptions			181.8
Effect of actual variances from expected experience	19.0		116.8
Adjusted beginning of period balance at original discount rate	5,023.1		4,882.4
Interest accrual	58.0		220.1
Benefit payments	(79.9)		(330.4)
Issuances	63.3		232.0
Balance at end of period at original discount rate	5,064.5		5,004.1
Effect of changes in discount rate assumptions at end of period	(236.2)		(124.5)
Balance at end of period, Present value of expected future policy benefit payments	4,828.3		4,879.6
Future policy benefits	1,092.2		1,085.9
Reinsurance Impact	40.4		45.2
Future policy benefits after reinsurance	1,132.6		1,131.1
Increase (decrease) to LFPB due to effect of changes in discount rate assumptions	$ (26.5)		$ 31.2
Weighted-average duration for future policy benefits (in years)	9 years 1 month 6 days		9 years 4 months 24 days
Benefits and Protection | Specialty Benefits | Individual disability
Present value of expected net premiums
Balance at beginning of period, Present value of expected net premiums	$ 2,552.3		$ 2,341.8
Effect of changes in discount rate assumptions at beginning of period	313.7		395.2
Balance at beginning of period at original discount rate	2,866.0		2,737.0
Effect of changes in cash flow assumptions			(37.6)
Effect of actual variances from expected experience	44.7		244.3
Adjusted beginning of period balance at original discount rate	2,910.7		2,943.7
Interest accrual	24.7		95.6
Net premiums collected	(68.5)		(273.4)
Issuances	19.8		100.1
Balance at end of period at original discount rate	2,886.7		2,866.0
Effect of changes in discount rate assumptions at end of period	(364.5)		(313.7)
Balance at end of period, Present value of expected net premiums	2,522.2		2,552.3
Present value of expected future policy benefit payments
Balance at beginning of period, Present value of expected future policy benefit payments	4,450.7		4,040.6
Effect of changes in discount rate assumptions at beginning of period	903.5		1,021.4
Balance at beginning of period at original discount rate	5,354.2		5,062.0
Effect of changes in cash flow assumptions			(51.5)
Effect of actual variances from expected experience	49.7		260.8
Adjusted beginning of period balance at original discount rate	5,403.9		5,271.3
Interest accrual	49.1		190.1
Benefit payments	(52.3)		(210.0)
Issuances	20.3		102.8
Balance at end of period at original discount rate	5,421.0		5,354.2
Effect of changes in discount rate assumptions at end of period	(1,043.9)		(903.5)
Balance at end of period, Present value of expected future policy benefit payments	4,377.1		4,450.7
Future policy benefits	1,854.9		1,898.4
Reinsurance Impact	(412.4)		(421.6)
Future policy benefits after reinsurance	1,442.5		1,476.8
Increase (decrease) to LFPB due to effect of changes in discount rate assumptions	$ (89.6)		$ 36.4
Increase (decrease) in LFPB from actuarial assumptions		$ (13.9)
Income before taxes, net of reinsurance increase (decrease) from actuarial assumptions		10.2
Weighted-average duration for future policy benefits (in years)	18 years 1 month 6 days		18 years 4 months 24 days
Benefits and Protection | Specialty Benefits | Term life
Present value of expected future policy benefit payments
Increase (decrease) in LFPB from actuarial assumptions		38.3
Income before taxes, net of reinsurance increase (decrease) from actuarial assumptions		$ (25.4)