Nature of Operations and Significant Accounting Policies - Balances and changes in the unearned revenue liability (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2022	Dec. 31, 2021
Change in unearned revenue liability
Beginning balance, Unearned Revenue Liability	$ 467.4
Ending balance, Unearned Revenue Liability	486.9	$ 467.4
Benefits and Protection | Life Insurance
Change in unearned revenue liability
Beginning balance, Unearned Revenue Liability			$ 384.4
Benefits and Protection | Life Insurance | Universal life
Change in unearned revenue liability
Beginning balance, Unearned Revenue Liability	459.0	425.3	384.4
Deferrals	42.9	61.8	66.1
Revenue recognized	(22.2)	(28.1)	(25.2)
Ending balance, Unearned Revenue Liability	$ 479.7	$ 459.0	$ 425.3