Nature of Operations and Significant Accounting Policies - Balances and changes in LFPB for Benefits and Protection and Corporate (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	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	$ 34,064.4		$ 43,489.8
Benefits and Protection
Present value of expected future policy benefit payments
Future policy benefits	2,554.2	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	(37.6)	(413.1)
Effect of actual variances from expected experience	204.1	235.9	122.0
Adjusted beginning of period balance at original discount rate	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	2,737.0	2,951.0
Effect of changes in discount rate assumptions at end of period	(485.4)	(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	(51.5)	(476.3)
Effect of actual variances from expected experience	218.7	240.0	118.8
Adjusted beginning of period balance at original discount rate	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	5,062.0	5,147.3
Effect of changes in discount rate assumptions at end of period	1,334.7	1,021.4	(501.0)
Balance at end of period, Present value of expected future policy benefit payments	3,970.3	4,040.6	5,648.3
Future policy benefits	1,604.5	1,698.8	2,498.4	2,500.9
Reinsurance Impact	(365.9)	(386.8)	(549.9)
Future policy benefits after reinsurance	1,238.6	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 changes in cash flow assumptions	143.5
Effect of actual variances from expected experience	92.7	128.6	211.4
Adjusted beginning of period balance at original discount rate	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	3,619.2	3,503.6
Effect of changes in discount rate assumptions at end of period	(362.4)	(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 changes in cash flow assumptions	181.8
Effect of actual variances from expected experience	106.9	139.0	216.0
Adjusted beginning of period balance at original discount rate	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	4,583.8	4,397.4
Effect of changes in discount rate assumptions at end of period	475.3	251.6	(913.9)
Balance at end of period, Present value of expected future policy benefit payments	4,461.9	4,332.2	5,311.3
Future policy benefits	949.7	909.0	1,117.6	1,160.3
Reinsurance Impact	40.7	25.6	(0.1)
Future policy benefits after reinsurance	990.4	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	(13.3)	10.0	2.8
Effect of actual variances from expected experience	(5.9)	4.1	(0.8)
Adjusted beginning of period balance at original discount rate	41.8	62.9	51.1
Interest accrual	2.2	3.3	2.9
Net premiums collected	(4.0)	(5.2)	(5.2)
Balance at end of period at original discount rate	40.0	61.0	48.8
Effect of changes in discount rate assumptions at end of period	0.8	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	(40.5)	17.2	0.5
Effect of actual variances from expected experience	2.0	3.7	(4.6)
Adjusted beginning of period balance at original discount rate	190.7	228.9	209.1
Interest accrual	9.6	13.1	12.4
Benefit payments	(10.4)	(12.8)	(13.5)
Balance at end of period at original discount rate	189.9	229.2	208.0
Effect of changes in discount rate assumptions at end of period	(3.9)	(18.9)	(89.5)
Balance at end of period, Present value of expected future policy benefit payments	193.8	248.1	297.5
Future policy benefits	153.0	183.5	234.7	$ 257.6
Reinsurance Impact	$ (153.0)	$ (183.5)	$ (234.7)