Deferred Acquisition Costs and Other Actuarial Balances - Disaggregated DAC amounts (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Deferred Acquisition Costs
Deferred acquisition costs	$ 3,948.4	$ 3,948.0		$ 3,822.3
Short-duration contracts
Deferred Acquisition Costs
Deferred acquisition costs	34.2	34.7
Other balances
Deferred Acquisition Costs
Deferred acquisition costs	7.7	8.8
Retirement and Income Solutions
Deferred Acquisition Costs
Deferred acquisition costs	920.5	930.0		964.4
Retirement and Income Solutions | Workplace savings and retirement solutions
Deferred Acquisition Costs
Deferred acquisition costs	504.1	498.0	$ 489.0	477.2
Retirement and Income Solutions | Individual variable annuities
Deferred Acquisition Costs
Deferred acquisition costs	277.7	278.0	281.2	281.9
Retirement and Income Solutions | Pension risk transfer
Deferred Acquisition Costs
Deferred acquisition costs	13.8	8.1	2.7
Retirement and Income Solutions | Individual fixed deferred annuities
Deferred Acquisition Costs
Deferred acquisition costs	112.2	131.0	162.2	192.7
Retirement and Income Solutions | Investment only
Deferred Acquisition Costs
Deferred acquisition costs	12.7	14.9	18.1	12.6
Benefits and Protection
Deferred Acquisition Costs
Deferred acquisition costs	2,986.0	2,974.5		2,857.9
Benefits and Protection | Life Insurance | Universal life
Deferred Acquisition Costs
Deferred acquisition costs	1,549.3	1,569.7	1,596.2	1,608.3
Benefits and Protection | Life Insurance | Term life
Deferred Acquisition Costs
Deferred acquisition costs	692.1	685.7	678.9	607.9
Benefits and Protection | Life Insurance | Participating life
Deferred Acquisition Costs
Deferred acquisition costs	86.6	93.0	102.3	111.9
Benefits and Protection | Specialty Benefits | Individual disability
Deferred Acquisition Costs
Deferred acquisition costs	$ 658.0	$ 626.1	$ 580.7	$ 529.8