Fair Values of Financial Instruments Level 3 Quantitative Sensitivity Analysis (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012		Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturity Securities	$ 42,411.2		$ 42,486.7
Equity Securities	12.6		11.2
States, Municipalities, and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturity Securities	1,556.4		1,591.6
Mortgage/Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturity Securities	2,842.3		2,973.2
All Other Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturity Securities	24,865.8		24,550.1
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturity Securities	1,399.5		1,141.4
Equity Securities	4.5		11.2
Fair Value, Inputs, Level 3 [Member] | States, Municipalities, and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturity Securities	66.4		68.1
Fair Value, Investments, Valuation Techniques	- Comparability Adjustment - Discount for Size - Volatility of Credit
Investments, Fair Value Disclosure, Significant Assumptions	0.50% - 1.25%/0.87% 0.20% - 0.20%/0.20% 0.25% - 0.25%/0.25%	[1],[2],[3]
Fair Value, Inputs, Level 3 [Member] | Mortgage/Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturity Securities	21.3		31.7
Fair Value, Investments, Valuation Techniques	- Discount for Size
Investments, Fair Value Disclosure, Significant Assumptions	4.99% - 5.08%/5.06%	[2]
Fair Value, Inputs, Level 3 [Member] | All Other Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturity Securities	733.2		665.5
Fair Value, Investments, Valuation Techniques	- Change in Benchmark Reference - Comparability Adjustment - Discount for Size - Lack of Marketability - Volatility of Credit - Market Convention
Investments, Fair Value Disclosure, Significant Assumptions	1.65% - 1.65%/1.65% 0.03% - 0.75%/0.50% 0.50% - 0.50%/0.50% 0.50% - 1.00%/0.74% (0.50)% - 7.72%/1.27% Priced at Par	[1],[2],[3],[4],[5],[6]
Fair Value, Inputs, Level 3 [Member] | Corporate Bond Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fair Value, Investments, Valuation Techniques	- Change in Benchmark Reference - Comparability Adjustment - Discount for Size
Investments, Fair Value Disclosure, Significant Assumptions	0.25% - 0.47%/0.27% 0.25% - 0.50%/0.41% 0.25% - 0.25%/0.25%	[1],[2],[4]
Fair Value, Inputs, Level 3 [Member] | Equity Securites
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fair Value, Investments, Valuation Techniques	- Market Convention
Investments, Fair Value Disclosure, Significant Assumptions	Priced at Cost or Owner's Equity	[6]
Fair Value, Inputs, Level 3 [Member] | Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fair Value, Derivative Financial Instruments, Liabilities, Valuation Techniques	- Projected Liability Cash Flows
Investments, Fair Value Disclosure, Significant Assumptions	Actuarial Assumptions	[7]
Estimate of Fair Value, Fair Value Disclosure [Member] | Fair Value, Inputs, Level 3 [Member] | States, Municipalities, and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturity Securities	66.4
Estimate of Fair Value, Fair Value Disclosure [Member] | Fair Value, Inputs, Level 3 [Member] | Mortgage/Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturity Securities	0.6
Estimate of Fair Value, Fair Value Disclosure [Member] | Fair Value, Inputs, Level 3 [Member] | All Other Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturity Securities	295.6
Estimate of Fair Value, Fair Value Disclosure [Member] | Fair Value, Inputs, Level 3 [Member] | Corporate Bond Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturity Securities	117.5
Estimate of Fair Value, Fair Value Disclosure [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Securites
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Equity Securities	4.0
Estimate of Fair Value, Fair Value Disclosure [Member] | Fair Value, Inputs, Level 3 [Member] | Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative Liability	$ (123.4)

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

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

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

[4] Represents basis point adjustments for changes in benchmark spreads associated with various ratings categories

[5] Represents basis point adjustments to apply a discount due to illiquidity

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

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