Quantitative Information Regarding Significant Unobservable Inputs (Details) $ in Millions		Sep. 30, 2021 USD ($) Rate	Dec. 31, 2020 USD ($) Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fixed Maturity Securities | $		$ 43,628.3	$ 44,137.3
Equity Securities | $		33.2	28.3
Public Utilities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fixed Maturity Securities | $		6,529.4	6,962.3
Corporate Debt Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fixed Maturity Securities | $		30,079.8	29,918.6
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 | $		$ (18.6)	$ (39.8)
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		0.006	0.010
Fair Value, Inputs, Level 3 | Corporate Debt Securities | Market Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
All Other Corporate Bonds - Private | $		$ 79.8	$ 45.7
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]	0.0002
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]	0.0606	0.0050
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]	0.0096
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]	0.0606	0.2490
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]	0.0029
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]	0.0606	0.0363
Fair Value, Inputs, Level 3 | Equity Securities | Market Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity Securities | $		$ 5.6	$ 4.7

[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