Quantitative Information Regarding Significant Unobservable Inputs (Details) $ in Millions		Sep. 30, 2020 USD ($) Rate		Dec. 31, 2019 USD ($) Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fixed Maturity Securities | $		$ 49,270.2		$ 47,443.7
Equity Securities | $		24.1		32.6
Public Utilities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fixed Maturity Securities | $		8,016.0		7,732.2
Corporate Debt Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fixed Maturity Securities | $		33,517.6		32,410.4
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 | $		$ (53.7)		$ (22.8)
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		0.013	[1]	0.008
Fair Value, Inputs, Level 3 | Public Utilities | Market Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fixed Maturity Securities | $		$ 111.6
Fair Value, Inputs, Level 3 | Public Utilities | Market Approach | Minimum | Lack of Marketability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
All Other Corporate Bonds - Private, Measurement Input	[2]	0.0012
Fair Value, Inputs, Level 3 | Public Utilities | Market Approach | Maximum | Lack of Marketability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
All Other Corporate Bonds - Private, Measurement Input	[2]	0.0127
Fair Value, Inputs, Level 3 | Public Utilities | Market Approach | Weighted Average | Lack of Marketability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
All Other Corporate Bonds - Private, Measurement Input	[2]	0.0076
Fair Value, Inputs, Level 3 | Corporate Debt Securities | Market Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
All Other Corporate Bonds - Private | $		$ 45.9		$ 119.2
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	[2]			0.0456
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	[3]	0.0050		0.0035
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	[2]			0.0456
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	[3]	0.1542		0.1768
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	[2]			0.0456
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	[3]	0.0243		0.022
Fair Value, Inputs, Level 3 | Equity Securities | Market Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity Securities | $		$ 4.6		$ 4.6

[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 basis point adjustments to apply a discount due to the illiquidity of an investmen

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