Quantitative Information Regarding Significant Unobservable Inputs (Details) - Fair Value, Inputs, Level 3 $ in Millions		Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Discounted Cash Flow | Embedded Derivative in Modified Coinsurance Arrangement
Fair Value Inputs, Assets and Liabilities, Quantitative Information
Embedded Derivative in Modified Coinsurance Arrangement		$ (13.8)	$ (15.9)
Embedded Derivative in Modified Coinsurance Arrangement, Measurement Input	[1]	us-gaap:ProjectedCashFlowsActuarialAssumptionsMember	us-gaap:ProjectedCashFlowsActuarialAssumptionsMember
All Other Corporate Bonds - Private | Market Approach
Fair Value Inputs, Assets and Liabilities, Quantitative Information
All Other Corporate Bonds - Private		$ 223.1	$ 244.4
All Other Corporate Bonds - Private, Market Convention Measurement Input	[2]	us-gaap:MarketConventionPricedatParMember	us-gaap:MarketConventionPricedatParMember
All Other Corporate Bonds - Private | Comparability Adjustment | Market Approach | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information
All Other Corporate Bonds - Private, Measurement Input	[3]	0.0069	0.0020
All Other Corporate Bonds - Private | Comparability Adjustment | Market Approach | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information
All Other Corporate Bonds - Private, Measurement Input	[3]	0.0069	0.0020
All Other Corporate Bonds - Private | Comparability Adjustment | Market Approach | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information
All Other Corporate Bonds - Private, Measurement Input	[3]	0.0069	0.0020
All Other Corporate Bonds - Private | Lack of Marketability | Market Approach | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information
All Other Corporate Bonds - Private, Measurement Input	[4]	0.0025	0.0025
All Other Corporate Bonds - Private | Lack of Marketability | Market Approach | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information
All Other Corporate Bonds - Private, Measurement Input	[4]	0.0025	0.0025
All Other Corporate Bonds - Private | Lack of Marketability | Market Approach | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information
All Other Corporate Bonds - Private, Measurement Input	[4]	0.0025	0.0025
All Other Corporate Bonds - Private | Volatility of Credit | Market Approach | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information
All Other Corporate Bonds - Private, Measurement Input	[5]	0.0014	0.0012
All Other Corporate Bonds - Private | Volatility of Credit | Market Approach | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information
All Other Corporate Bonds - Private, Measurement Input	[5]	0.0556	0.0625
All Other Corporate Bonds - Private | Volatility of Credit | Market Approach | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information
All Other Corporate Bonds - Private, Measurement Input	[5]	0.0056	0.0050
Equity Securities - Private | Market Approach
Fair Value Inputs, Assets and Liabilities, Quantitative Information
Equity Securities - Private		$ 1.1	$ 1.1
Equity Securities - Private, Measurement Input	[2]	us-gaap:MarketConventionPricedatCostorOwnersEquityMember	us-gaap:MarketConventionPricedatCostorOwnersEquityMember

[1] Represents various actuarial assumptions required to derive the liability cash flows including incidence, termination, and lapse rates

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

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

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

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