Nature of Operations and Significant Accounting Policies - Cumulative effect adjustment on consolidated statements of financial position (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022	Dec. 31, 2020
Assets
Other investments	$ 6,569.2	$ 6,122.0
Reinsurance recoverable and deposit receivable	20,219.1	21,154.0
Premiums due and other receivables	3,897.0	3,933.3
Deferred acquisition costs	3,948.4	3,948.0	$ 3,822.3
Market risk benefit asset	173.1	109.2
Other assets	833.9	1,205.6
Total assets	287,948.0	290,578.3
Liabilities
Contractholder funds	42,303.9	42,825.3
Future policy benefits and claims	42,685.9	43,025.3
Market risk benefit liability	112.5	207.4
Other policyholder funds	946.0	911.5
Deferred income taxes	1,481.4	1,324.2
Total liabilities	276,965.6	280,298.5
Stockholders' equity
Retained earnings	17,717.0	16,697.3
Accumulated other comprehensive income (loss)	(6,840.9)	(6,879.0)
Total stockholders' equity	10,671.6	9,976.7
Total liabilities and stockholders' equity	$ 287,948.0	$ 290,578.3
Effects of implementation of accounting change | ASU 2018-12 - Targeted improvements to the accounting for long-duration insurance contracts
Assets
Other investments			(1.6)
Reinsurance recoverable and deposit receivable			187.7
Premiums due and other receivables			(24.4)
Deferred acquisition costs			450.9
Market risk benefit asset			18.9
Other assets			19.3
Total assets			650.8
Liabilities
Contractholder funds			(397.9)
Future policy benefits and claims			7,143.4
Market risk benefit liability			663.5
Other policyholder funds			48.8
Deferred income taxes			(1,519.1)
Total liabilities			5,938.7
Stockholders' equity
Retained earnings			(159.9)
Accumulated other comprehensive income (loss)			(5,128.0)
Total stockholders' equity			(5,287.9)
Total liabilities and stockholders' equity			$ 650.8