Nature of Operations and Significant Accounting Policies - Balances and changes in Additional Liability for Certain Benefit Features (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2022	Dec. 31, 2021
Changes in the additional liability for certain benefits features
Balance at end of period	$ 5,176.7	$ 4,095.2
Benefits and Protection | Life Insurance | Universal life
Changes in the additional liability for certain benefits features
Balance at beginning of period	4,095.2	3,814.2	$ 3,463.9
Effect of changes in cash flow assumptions	725.4	(6.0)	(11.1)
Effect of actual variances from expected experience	27.7	54.9	18.1
Interest accrual	150.6	171.2	158.3
Net assessments collected	277.1	320.3	324.7
Benefit payments	(99.3)	(91.9)	(83.1)
Other		(167.5)	(56.6)
Balance at end of period	$ 5,176.7	$ 4,095.2	$ 3,814.2