Nature of Operations and Significant Accounting Policies - Balances and changes in LFPB for Benefits and Protection and Corporate (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2023	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 | Specialty Benefits | Individual disability
Present value of expected net premiums
Balance at beginning of period, Present value of expected net premiums		3,149.9	$ 3,148.3
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, Present value of expected net premiums			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
Benefits and Protection | Life Insurance | Term life
Present value of expected net premiums
Balance at beginning of period, Present value of expected net premiums		4,193.7	3,799.0
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, Present value of expected net premiums			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
Corporate | Long-term care insurance
Present value of expected net premiums
Balance at beginning of period, Present value of expected net premiums		62.8	66.7
Effect of changes in discount rate assumptions at beginning of period	(3.6)	(14.0)	(17.6)
Balance at beginning of period at original discount rate	61.0	48.8	49.1
Effect of changes in cash flow assumptions		10.0	2.8
Effect of actual variances from expected experience	(3.6)	4.1	(0.8)
Adjusted beginning of period balance at original discount rate	57.4	62.9	51.1
Interest accrual	0.9	3.3	2.9
Net premiums collected	(1.5)	(5.2)	(5.2)
Balance at end of period at original discount rate	56.8	61.0	48.8
Effect of changes in discount rate assumptions at end of period	4.7	3.6	14.0
Balance at end of period, Present value of expected net premiums			62.8
Present value of expected future policy benefit payments
Balance at beginning of period, Present value of expected future policy benefit payments	248.1	297.5	324.3
Effect of changes in discount rate assumptions at beginning of period	(18.9)	(89.5)	(111.1)
Balance at beginning of period at original discount rate	229.2	208.0	213.2
Effect of changes in cash flow assumptions		17.2	0.5
Effect of actual variances from expected experience	(5.9)	3.7	(4.6)
Adjusted beginning of period balance at original discount rate	223.3	228.9	209.1
Interest accrual	3.4	13.1	12.4
Benefit payments	(3.3)	(12.8)	(13.5)
Balance at end of period at original discount rate	223.4	229.2	208.0
Effect of changes in discount rate assumptions at end of period	(26.5)	(18.9)	(89.5)
Balance at end of period, Present value of expected future policy benefit payments	249.9	248.1	297.5
Future policy benefits	188.4	183.5	234.7	$ 257.6
Reinsurance Impact	$ (188.4)	$ (183.5)	$ (234.7)