Future Policy Benefits and Claims - Benefits and Protection (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Present value of expected future policy benefit payments
Future policy benefits	$ 32,949.4		$ 32,949.4	$ 34,064.4		$ 43,489.8
Benefits and Protection
Present value of expected future policy benefit payments
Future policy benefits	2,554.2		2,554.2	2,607.8		3,661.2
Benefits and Protection | Life Insurance | Term life
Present value of expected net premiums
Balance at beginning of period, Present value of expected net premiums			3,423.2	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 changes in cash flow assumptions	143.5		143.5
Effect of actual variances from expected experience	92.7		92.7	128.6	211.4
Adjusted beginning of period balance at original discount rate	3,855.4		3,855.4	3,632.2	3,149.3
Interest accrual			127.5	162.9	151.1
Net premiums collected			(266.4)	(346.6)	(315.0)
Issuances			158.1	170.7	518.2
Balance at end of period at original discount rate	3,874.6		3,874.6	3,619.2	3,503.6
Effect of changes in discount rate assumptions at end of period	(362.4)		(362.4)	(196.0)	690.1
Balance at end of period, Present value of expected net premiums	3,512.2		3,512.2	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 changes in cash flow assumptions	181.8		181.8
Effect of actual variances from expected experience	106.9		106.9	139.0	216.0
Adjusted beginning of period balance at original discount rate	4,872.5		4,872.5	4,536.4	3,995.4
Interest accrual			162.9	206.8	194.3
Benefit payments			(268.2)	(340.6)	(335.7)
Issuances			170.0	181.2	543.4
Balance at end of period at original discount rate	4,937.2		4,937.2	4,583.8	4,397.4
Effect of changes in discount rate assumptions at end of period	(475.3)		(475.3)	(251.6)	913.9
Balance at end of period, Present value of expected future policy benefit payments	4,461.9		4,461.9	4,332.2	5,311.3
Future policy benefits	949.7		949.7	909.0	1,117.6	1,160.3
Reinsurance Impact	40.7		40.7	25.6	(0.1)
Future policy benefits after reinsurance	$ 990.4		990.4	934.6	1,117.5
Liability for future policy benefit increase (decrease) due to effect of changes in discount rate assumptions			$ (57.3)	$ (279.4)
Weighted-average duration for future policy benefits (in years)	9 years 6 months		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, Present value of expected net premiums			$ 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	$ (37.6)		(37.6)	(413.1)
Effect of actual variances from expected experience	204.1		204.1	235.9	122.0
Adjusted beginning of period balance at original discount rate	2,903.5		2,903.5	2,773.8	2,929.6
Interest accrual			71.3	95.2	95.4
Net premiums collected			(204.7)	(276.5)	(280.4)
Issuances			81.1	144.5	206.4
Balance at end of period at original discount rate	2,851.2		2,851.2	2,737.0	2,951.0
Effect of changes in discount rate assumptions at end of period	(485.4)		(485.4)	(395.2)	198.9
Balance at end of period, Present value of expected net premiums	2,365.8		2,365.8	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	(51.5)		(51.5)	(476.3)
Effect of actual variances from expected experience	218.7		218.7	240.0	118.8
Adjusted beginning of period balance at original discount rate	5,229.2		5,229.2	4,911.0	4,933.9
Interest accrual			141.8	185.4	181.0
Benefit payments			(149.6)	(183.7)	(179.7)
Issuances			83.6	149.3	212.1
Balance at end of period at original discount rate	5,305.0		5,305.0	5,062.0	5,147.3
Effect of changes in discount rate assumptions at end of period	(1,334.7)		(1,334.7)	(1,021.4)	501.0
Balance at end of period, Present value of expected future policy benefit payments	3,970.3		3,970.3	4,040.6	5,648.3
Future policy benefits	1,604.5		1,604.5	1,698.8	2,498.4	$ 2,500.9
Reinsurance Impact	(365.9)		(365.9)	(386.8)	(549.9)
Future policy benefits after reinsurance	1,238.6		1,238.6	1,312.0	$ 1,948.5
Liability for future policy benefit increase (decrease) due to effect of changes in discount rate assumptions			$ (223.1)	$ (928.3)
Liability for future policy benefits increase (decrease) from actuarial assumptions	(13.9)	$ (63.2)
Pre-tax operating earnings, net of reinsurance, increase (decrease) from actuarial assumptions	$ 10.2	$ 52.4
Weighted-average duration for future policy benefits (in years)	18 years 3 months 18 days		18 years 3 months 18 days	18 years 6 months
Benefits and Protection | Specialty Benefits | Term life
Present value of expected future policy benefit payments
Liability for future policy benefits increase (decrease) from actuarial assumptions	$ 38.3
Pre-tax operating earnings, net of reinsurance, increase (decrease) from actuarial assumptions	$ (25.4)