Nature of Operations and Significant Accounting Policies - Balances and changes in DAC (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2022	Dec. 31, 2021
Change in deferred acquisition costs
Beginning balance, Deferred acquisition costs	$ 3,948.0		$ 3,822.3
Ending balance, Deferred acquisition costs	3,948.4	$ 3,948.0
Retirement and Income Solutions
Change in deferred acquisition costs
Beginning balance, Deferred acquisition costs	930.0		964.4
Ending balance, Deferred acquisition costs	920.5	930.0
Benefits and Protection
Change in deferred acquisition costs
Beginning balance, Deferred acquisition costs	2,974.5		2,857.9
Ending balance, Deferred acquisition costs	2,986.0	2,974.5
Workplace savings and retirement solutions | Retirement and Income Solutions
Change in deferred acquisition costs
Beginning balance, Deferred acquisition costs	498.0	489.0	477.2
Costs deferred	35.7	48.3	50.1
Amortized to expense	(29.6)	(39.3)	(38.3)
Ending balance, Deferred acquisition costs	504.1	498.0	489.0
Individual variable annuities | Retirement and Income Solutions
Change in deferred acquisition costs
Beginning balance, Deferred acquisition costs	278.0	281.2	281.9
Costs deferred	19.1	22.5	26.4
Amortized to expense	(19.4)	(25.7)	(27.1)
Ending balance, Deferred acquisition costs	277.7	278.0	281.2
Pension risk transfer | Retirement and Income Solutions
Change in deferred acquisition costs
Beginning balance, Deferred acquisition costs	8.1	2.7
Costs deferred	6.1	5.6	2.7
Amortized to expense	(0.4)	(0.2)
Ending balance, Deferred acquisition costs	13.8	8.1	2.7
Individual fixed deferred annuities | Retirement and Income Solutions
Change in deferred acquisition costs
Beginning balance, Deferred acquisition costs	131.0	162.2	192.7
Costs deferred			7.6
Amortized to expense	(18.8)	(31.2)	(38.1)
Ending balance, Deferred acquisition costs	112.2	131.0	162.2
Investment only | Retirement and Income Solutions
Change in deferred acquisition costs
Beginning balance, Deferred acquisition costs	14.9	18.1	12.6
Costs deferred	1.4	1.4	9.1
Amortized to expense	(3.6)	(4.6)	(3.6)
Ending balance, Deferred acquisition costs	12.7	14.9	18.1
Individual disability | Benefits and Protection | Specialty Benefits
Change in deferred acquisition costs
Beginning balance, Deferred acquisition costs	626.1	580.7	529.8
Costs deferred	65.4	87.6	89.8
Amortized to expense	(33.5)	(42.2)	(38.9)
Ending balance, Deferred acquisition costs	658.0	626.1	580.7
Universal life | Benefits and Protection | Life Insurance
Change in deferred acquisition costs
Beginning balance, Deferred acquisition costs	1,569.7	1,596.2	1,608.3
Costs deferred	51.4	70.6	84.3
Amortized to expense	(71.8)	(97.1)	(96.4)
Ending balance, Deferred acquisition costs	1,549.3	1,569.7	1,596.2
Term life | Benefits and Protection | Life Insurance
Change in deferred acquisition costs
Beginning balance, Deferred acquisition costs	685.7	678.9	607.9
Costs deferred	52.5	66.7	126.5
Amortized to expense	(46.1)	(59.9)	(55.5)
Ending balance, Deferred acquisition costs	692.1	685.7	678.9
Participating life | Benefits and Protection | Life Insurance
Change in deferred acquisition costs
Beginning balance, Deferred acquisition costs	93.0	102.3	111.9
Costs deferred	1.0	1.4	1.9
Amortized to expense	(7.4)	(10.7)	(11.5)
Ending balance, Deferred acquisition costs	$ 86.6	$ 93.0	$ 102.3