Basis of Presentation and Accounting Policies - AOCI Effects of Adoption (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022	Dec. 31, 2020
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained Earnings	$ 982.4	$ 1,366.4
Accumulated Other Comprehensive Loss	$ (473.4)	$ (514.9)
Effect of Retrospective Application of Accounting Standards Update 2018-12 | Pre Adoption Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained Earnings			$ 2,071.2
Accumulated Other Comprehensive Loss			680.5
Effect of Retrospective Application of Accounting Standards Update 2018-12 | Adjustments to AOCI
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained Earnings			0.0
Accumulated Other Comprehensive Loss			(1,030.3)
Effect of Retrospective Application of Accounting Standards Update 2018-12 | Adjustments to Retained Earnings for Net Pemiums
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained Earnings			(25.1)
Accumulated Other Comprehensive Loss			0.0
Effect of Retrospective Application of Accounting Standards Update 2018-12 | Post Adoption Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained Earnings			2,046.1
Accumulated Other Comprehensive Loss			$ (349.8)