Basis of Presentation and Significant Accounting Policies (Details) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2025 USD ($)	Jun. 30, 2025 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2025 USD ($) segment	Sep. 30, 2024 USD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of reportable segments | segment				4
Net income	$ (58.3)		$ (34.3)	$ (125.9)	$ (64.6)
Net investment income, pre-tax	(119.6)		(113.0)	(346.3)	(326.8)
Life & Retirement
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income	(16.9)		(14.8)	(43.7)	(38.8)
Supplemental & Group Benefits
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income	$ (15.0)		(14.8)	$ (34.1)	(39.9)
Revision of Prior Period, Error Correction, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income		$ 8.1	$ 0.8		$ 3.3	$ 4.4	$ 1.3
Net investment income, pre-tax		10.2
Revision of Prior Period, Error Correction, Adjustment | Life & Retirement
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income		5.3
Revision of Prior Period, Error Correction, Adjustment | Supplemental & Group Benefits
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income		$ 2.8