Quantitative Information Regarding Significant Unobservable Inputs (Details) $ in Millions		Jun. 30, 2020 USD ($) Rate	Dec. 31, 2019 USD ($) Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity Securities | $		$ 24.8	$ 32.6
Fixed Maturity Securities | $		48,233.4	47,443.7
Public Utilities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fixed Maturity Securities | $		7,923.0	7,732.2
Corporate Debt Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fixed Maturity Securities | $		32,915.2	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 | $	[1]	(67.8)	(22.8)
Fair Value, Inputs, Level 3 | Public Utilities | Market Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fixed Maturity Securities | $		$ 100.3
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]	0.41
Fair Value, Inputs, Level 3 | Public Utilities | Market Approach | Minimum | Measurement Input, Discount for Size
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
All Other Corporate Bonds - Private, Measurement Input	[3]	0.07
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]	1.85
Fair Value, Inputs, Level 3 | Public Utilities | Market Approach | Maximum | Measurement Input, Discount for Size
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
All Other Corporate Bonds - Private, Measurement Input	[3]	0.07
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]	0.79
Fair Value, Inputs, Level 3 | Public Utilities | Market Approach | Weighted Average | Measurement Input, Discount for Size
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
All Other Corporate Bonds - Private, Measurement Input	[3]	0.07
Fair Value, Inputs, Level 3 | Corporate Debt Securities | Market Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
All Other Corporate Bonds - Private | $	[2],[4],[5]	$ 70.7	$ 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]	0.0020	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	[5]	0.0050	0.0035
Fair Value, Inputs, Level 3 | Corporate Debt Securities | Market Approach | Minimum | Measurement Input, Comparability Adjustment
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
All Other Corporate Bonds - Private, Measurement Input	[6]	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	[2]	0.0139	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	[5]	0.1244	0.1768
Fair Value, Inputs, Level 3 | Corporate Debt Securities | Market Approach | Maximum | Measurement Input, Comparability Adjustment
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
All Other Corporate Bonds - Private, Measurement Input	[6]	0.0050
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]	0.0099	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	[5]	0.0353	0.022
Fair Value, Inputs, Level 3 | Corporate Debt Securities | Market Approach | Weighted Average | Measurement Input, Comparability Adjustment
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
All Other Corporate Bonds - Private, Measurement Input		0.0050
Fair Value, Inputs, Level 3 | Equity Securities | Market Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity Securities | $	[4]	$ 4.5	$ 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. (e)Represents basis point adjustments based on issue/issuer size relative to the benchmark (f)Represents basis point adjustments for changes in benchmark spreads associated with various industry sectors

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

[3] Represents basis point adjustments based on issue/issuer size relative to the benchmark

[4] Represents a decision to price based on par value, cost, or owner's equity when limited data is available

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

[6] Represents basis point adjustments for changes in benchmark spreads associated with various industry sectors