Fair Value OF Financial Instrument - Quantitative Information about Significant Unobservable Inputs for Level 3 Recurring Fair Value Measurements (Details) $ in Millions	Sep. 30, 2025 USD ($)	Jun. 30, 2025 USD ($)	Dec. 31, 2024 USD ($)	Sep. 30, 2024 USD ($)	Jun. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 5,666.5		$ 5,387.9
Net MRBs	(9.3)	$ (6.7)	(6.8)	$ (5.7)	$ (6.2)	$ (3.9)
Corporate bonds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	1,840.7		1,782.4
Fair Value | Fair Value, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	5,666.5		5,387.9
Fair Value | Fair Value, Recurring | Corporate bonds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 1,840.8		1,782.4
Level 3 | Minimum | option budget | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivatives embedded in fixed indexed annuity products, measurement input	0.009
Level 3 | Maximum | option budget | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivatives embedded in fixed indexed annuity products, measurement input	0.038
Level 3 | Weighted Average | lapse rate | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivatives embedded in fixed indexed annuity products, measurement input	0.075
Net MRBs, measurement input	0.075
Level 3 | Weighted Average | mortality multiplier | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivatives embedded in fixed indexed annuity products, measurement input	0.710
Net MRBs, measurement input	0.710
Level 3 | Weighted Average | non-performance adjustment | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivatives embedded in fixed indexed annuity products, measurement input	0.050
Level 3 | Corporate bonds | Minimum | yield | discounted cash flow | Internal
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Corporate bonds, measurement input	0.032
Level 3 | Corporate bonds | Minimum | option adjusted spread | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Corporate bonds, measurement input	0.0255
Level 3 | Corporate bonds | Maximum | yield | discounted cash flow | Internal
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Corporate bonds, measurement input	0.088
Level 3 | Corporate bonds | Maximum | option adjusted spread | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Corporate bonds, measurement input	0.0781
Level 3 | Fair Value | Fair Value, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 477.4		499.9
Derivatives embedded in fixed indexed annuity products	79.1
Net MRBs	(9.3)
Level 3 | Fair Value | Fair Value, Recurring | Internal
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	193.6		199.3
Level 3 | Fair Value | Fair Value, Recurring | Corporate bonds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	340.2		351.3
Level 3 | Fair Value | Fair Value, Recurring | Corporate bonds | Internal
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 193.6		$ 199.3