Future Policy Benefits and Claims - Retirement and Income Solutions (Details) - Retirement and Income Solutions - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2022	Dec. 31, 2021
Pension risk transfer
Present value of expected future policy benefit payments
Balance at beginning of period, Present value of expected future policy benefit payments	$ 21,211.4	$ 25,365.8	$ 25,863.7
Effect of changes in discount rate assumptions at beginning of period	1,799.6	(3,386.5)	(4,827.9)
Balance at beginning of period at original discount rate	23,011.0	21,979.3	21,035.8
Effect of changes in cash flow assumptions	(53.4)	(7.9)
Effect of actual variances from expected experience	(9.8)	(3.2)	(2.4)
Adjusted beginning of period balance at original discount rate	22,947.8	21,968.2	21,033.4
Interest accrual	749.5	935.7	916.7
Benefit payments	(1,471.6)	(1,841.7)	(1,772.5)
Issuances	1,747.7	1,948.8	1,801.7
Balance at end of period at original discount rate	23,973.4	23,011.0	21,979.3
Effect of changes in discount rate assumptions at end of period	(2,618.2)	(1,799.6)	3,386.5
Balance at end of period, Present value of expected future policy benefit payments	21,355.2	21,211.4	25,365.8
Future policy benefits after reinsurance	$ 21,355.2	$ 21,211.4
Weighted-average duration for future policy benefits (in years)	8 years	8 years 6 months
Liability for future policy benefit increase (decrease) due to effect of changes in discount rate assumptions	$ (818.6)	$ (5,186.1)
Individual fixed income annuities
Present value of expected future policy benefit payments
Balance at beginning of period, Present value of expected future policy benefit payments	5,019.4	6,535.0	7,145.8
Effect of changes in discount rate assumptions at beginning of period	439.0	(812.6)	(1,213.5)
Balance at beginning of period at original discount rate	5,458.4	5,722.4	5,932.3
Effect of changes in cash flow assumptions	(1.3)	(3.0)
Effect of actual variances from expected experience	(0.7)		(1.3)
Adjusted beginning of period balance at original discount rate	5,456.4	5,719.4	5,931.0
Interest accrual	165.3	229.7	240.5
Benefit payments	(378.5)	(520.4)	(532.0)
Issuances	31.2	29.7	82.9
Balance at end of period at original discount rate	5,274.4	5,458.4	5,722.4
Effect of changes in discount rate assumptions at end of period	(612.5)	(439.0)	812.6
Balance at end of period, Present value of expected future policy benefit payments	4,661.9	5,019.4	$ 6,535.0
Reinsurance Impact	(4,344.7)	(4,714.0)
Future policy benefits after reinsurance	$ 317.2	$ 305.4
Weighted-average duration for future policy benefits (in years)	7 years 7 months 6 days	7 years 10 months 24 days
Liability for future policy benefit increase (decrease) due to effect of changes in discount rate assumptions	$ (173.5)	$ (1,251.6)