Deferred Acquisition Costs and Other Actuarial Balances - Benefits and Protection - Balances and changes in DAC (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2022	Dec. 31, 2021
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
Benefits and Protection
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
Individual disability | Benefits and Protection | Specialty Benefits
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
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
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
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