Quantitative Information Regarding Significant Unobservable Inputs (Details) $ in Millions		Sep. 30, 2022 USD ($) Rate	Dec. 31, 2021 USD ($) Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fixed Maturity Securities		$ 34,148.2	$ 43,336.0
Equity Securities		40.2	33.7
Public Utilities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fixed Maturity Securities		4,910.9	6,416.0
Corporate Debt Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fixed Maturity Securities		23,792.6	29,823.4
Mortgage/Asset-backed Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fixed Maturity Securities		530.3	638.3
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 in Modified Coinsurance Arrangement		$ (19.4)	$ (30.1)
Fair Value, Inputs, Level 3 | Discounted Cash Flow | Embedded Derivative in Modified Coinsurance Arrangement | Weighted Average | Spread of Swap Curve
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Embedded Derivative Liability, Measurement Input | Rate		0.007	0.007
Fair Value, Inputs, Level 3 | Public Utilities | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fixed Maturity Securities		$ 13.2
Fair Value, Inputs, Level 3 | Corporate Debt Securities | Market Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
All Other Corporate Bonds - Private		$ 15.9	$ 111.8
Fair Value, Inputs, Level 3 | Corporate Debt Securities | Market Approach | Minimum | Lack of Marketability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
All Other Corporate Bonds - Private, Measurement Input | Rate	[1]		0.0014
Fair Value, Inputs, Level 3 | Corporate Debt Securities | Market Approach | Minimum | Volatility of Credit
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
All Other Corporate Bonds - Private, Measurement Input | Rate	[2]	0.0461	0.0630
Fair Value, Inputs, Level 3 | Corporate Debt Securities | Market Approach | Maximum | Lack of Marketability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
All Other Corporate Bonds - Private, Measurement Input | Rate	[1]		0.0073
Fair Value, Inputs, Level 3 | Corporate Debt Securities | Market Approach | Maximum | Volatility of Credit
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
All Other Corporate Bonds - Private, Measurement Input | Rate	[2]	0.0461	0.0630
Fair Value, Inputs, Level 3 | Corporate Debt Securities | Market Approach | Weighted Average | Lack of Marketability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
All Other Corporate Bonds - Private, Measurement Input | Rate	[1]		0.0051
Fair Value, Inputs, Level 3 | Corporate Debt Securities | Market Approach | Weighted Average | Volatility of Credit
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
All Other Corporate Bonds - Private, Measurement Input | Rate	[2]	0.0461	0.0630
Fair Value, Inputs, Level 3 | Equity Securities | Market Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity Securities		$ 13.5	$ 5.8
Fair Value, Inputs, Level 3 | Mortgage/Asset-backed Securities | Market Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fixed Maturity Securities		$ 9.7

[1] Represents basis point adjustments to apply a discount due to the illiquidity of an investment

[2] Represents basis point adjustments for credit-specific factors