Future Policy Benefits and Claims - Benefits and Protection (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2023	Sep. 30, 2022	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Present value of expected future policy benefit payments
Future policy benefits	$ 35,620.8		$ 34,064.4		$ 43,489.8
Benefits and Protection
Present value of expected future policy benefit payments
Future policy benefits	2,777.1		2,607.8		3,661.2
Benefits and Protection | Life Insurance | Term life
Present value of expected net premiums
Balance at beginning of period	3,423.2		4,193.7
Effect of changes in discount rate assumptions at beginning of period	196.0		(690.1)	$ (861.1)
Balance at beginning of period at original discount rate	3,619.2		3,503.6	2,937.9
Effect of actual variances from expected experience	28.4		128.6	211.4
Adjusted beginning of period balance at original discount rate	3,647.6		3,632.2	3,149.3
Interest accrual	41.5		162.9	151.1
Net premiums collected	(86.4)		(346.6)	(315.0)
Issuances	49.6		170.7	518.2
Balance at end of period at original discount rate	3,652.3		3,619.2	3,503.6
Effect of changes in discount rate assumptions at end of period	(95.9)		(196.0)	690.1
Balance at end of period	3,556.4		3,423.2	4,193.7
Present value of expected future policy benefit payments
Balance at beginning of period, Present value of expected future policy benefit payments	4,332.2		5,311.3	4,959.3
Effect of changes in discount rate assumptions at beginning of period	251.6		(913.9)	(1,179.9)
Balance at beginning of period at original discount rate	4,583.8		4,397.4	3,779.4
Effect of actual variances from expected experience	32.2		139.0	216.0
Adjusted beginning of period balance at original discount rate	4,616.0		4,536.4	3,995.4
Interest accrual	52.9		206.8	194.3
Benefit payments	(87.3)		(340.6)	(335.7)
Issuances	53.5		181.2	543.4
Balance at end of period at original discount rate	4,635.1		4,583.8	4,397.4
Effect of changes in discount rate assumptions at end of period	(117.5)		(251.6)	913.9
Balance at end of period, Present value of expected future policy benefit payments	4,517.6		4,332.2	5,311.3
Future policy benefits	961.2		909.0	1,117.6	1,160.3
Reinsurance Impact	43.5		25.6	(0.1)
Future policy benefits after reinsurance	1,004.7		934.6	1,117.5
Liability for future policy benefit increase (decrease) due to effect of changes in discount rate assumptions	$ 34.0		$ (279.4)
Weighted-average duration for future policy benefits (in years)	9 years 6 months		9 years 7 months 6 days
Benefits and Protection | Specialty Benefits | Individual disability
Present value of expected net premiums
Balance at beginning of period	$ 2,341.8		$ 3,149.9
Effect of changes in discount rate assumptions at beginning of period	395.2		(198.9)	(340.7)
Balance at beginning of period at original discount rate	2,737.0		2,951.0	2,807.6
Effect of changes in cash flow assumptions			(413.1)
Effect of actual variances from expected experience	82.9		235.9	122.0
Adjusted beginning of period balance at original discount rate	2,819.9		2,773.8	2,929.6
Interest accrual	23.5		95.2	95.4
Net premiums collected	(66.1)		(276.5)	(280.4)
Issuances	23.5		144.5	206.4
Balance at end of period at original discount rate	2,800.8		2,737.0	2,951.0
Effect of changes in discount rate assumptions at end of period	(315.0)		(395.2)	198.9
Balance at end of period	2,485.8		2,341.8	3,149.9
Present value of expected future policy benefit payments
Balance at beginning of period, Present value of expected future policy benefit payments	4,040.6		5,648.3	5,649.2
Effect of changes in discount rate assumptions at beginning of period	1,021.4		(501.0)	(834.1)
Balance at beginning of period at original discount rate	5,062.0		5,147.3	4,815.1
Effect of changes in cash flow assumptions			(476.3)
Effect of actual variances from expected experience	92.4		240.0	118.8
Adjusted beginning of period balance at original discount rate	5,154.4		4,911.0	4,933.9
Interest accrual	46.7		185.4	181.0
Benefit payments	(49.3)		(183.7)	(179.7)
Issuances	24.3		149.3	212.1
Balance at end of period at original discount rate	5,176.1		5,062.0	5,147.3
Effect of changes in discount rate assumptions at end of period	(874.4)		(1,021.4)	501.0
Balance at end of period, Present value of expected future policy benefit payments	4,301.7		4,040.6	5,648.3
Future policy benefits	1,815.9		1,698.8	2,498.4	$ 2,500.9
Reinsurance Impact	(412.3)		(386.8)	(549.9)
Future policy benefits after reinsurance	1,403.6		1,312.0	$ 1,948.5
Liability for future policy benefit increase (decrease) due to effect of changes in discount rate assumptions	$ 66.8		$ (928.3)
Liability for future policy benefits increase (decrease) from actuarial assumptions		$ (63.2)
Pre-tax operating earnings, net of reinsurance, increase (decrease) from actuarial assumptions		$ 52.4
Weighted-average duration for future policy benefits (in years)	18 years 6 months		18 years 6 months