Basis of Presentation and Significant Accounting Policies - Schedule of Changes in the Statement of Equity due to Adoption of ASU 2018-12 (Details) - USD ($) $ in Millions	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021	Dec. 31, 2020
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity	$ 1,051.3		$ 1,098.3	$ 1,092.3
AOCI
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity	(409.9)	$ (359.6)	(399.4)	(433.2)	$ (328.9)	$ (50.0)
AOCI | Liability for future policy benefits
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity							$ (496.3)
AOCI | Deferred policy acquisition costs
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity							71.1
AOCI | Deferred sales inducements
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity							0.0
AOCI | Market risk benefits
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity							(1.3)
AOCI | Revision of Prior Period, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity							(426.5)
Retained earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity	$ 1,476.5	$ 1,478.6	$ 1,512.4	$ 1,542.4	$ 1,535.3	$ 1,547.0
Retained earnings | Liability for future policy benefits
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity							(0.2)
Retained earnings | Deferred policy acquisition costs
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity							0.0
Retained earnings | Deferred sales inducements
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity							0.0
Retained earnings | Market risk benefits
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity							(5.4)
Retained earnings | Revision of Prior Period, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity							$ (5.6)