Basis of Presentation and Significant Accounting Policies - Schedule of Changes in MRBs 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]
Market risk benefit	$ (4.5)	$ (3.6)	$ 0.2	$ 2.9	$ 4.6	$ 4.8	$ 8.6
Less: Reinsurance recoverable							0.0
Market risk benefits	$ 37.6			$ 79.9			8.6
Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Market risk benefit							0.1
Total adjustment for the difference between carrying amount and fair value
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Market risk benefit							8.5
Adjustment for the difference between carrying amount and fair value, except for the difference due to instrument-specific credit risk
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Market risk benefit							6.8
Adjustment for cumulative effect of changes in the instrument-specific credit risk at issuance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Market risk benefit							$ 1.7