Quantitative Information Regarding Significant Unobservable Inputs (Details) $ in Millions		Mar. 31, 2026 USD ($)	Dec. 31, 2025 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt Securities, Available-for-Sale		$ 33,300.8	$ 33,056.6
Equity Securities		20.0	20.0
All Other Corporate Bonds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt Securities, Available-for-Sale		22,540.9	22,462.1
Fair Value, Inputs, Level 3 | Discounted Cash Flow | Embedded Derivative in Modified Coinsurance Arrangement
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative Asset	[1]	$ 14.8	$ 17.8
Fair Value, Inputs, Level 3 | Discounted Cash Flow | Weighted Average | Input Measurement, Spread of Swap Curve | Embedded Derivative in Modified Coinsurance Arrangement
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Embedded Derivative Liability, Measurement Input		(0.0034)	(0.0041)
Fair Value, Inputs, Level 3 | All Other Corporate Bonds | Market Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt Securities, Available-for-Sale	[2]	$ 5.7	$ 8.3
Fair Value, Inputs, Level 3 | Perpetual Preferred and Equity Securities | Market Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity Securities	[2]	$ 19.8	$ 19.8

[1] Represents various actuarial assumptions required to derive the liability cash flows. Fair value of embedded derivative is most often driven by the change in the weighted average credit spread to the swap curve for the assets backing the hypothetical loan

[2] Represents a decision to price based on par value, cost, owner's equity, or the price of the most recent capital funding round when limited data is available