Fair Value of Financial Instruments - Quantitative Estimates for Level 3 Assets (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended		12 Months Ended
	Jun. 30, 2014		Dec. 31, 2013
Market Approach Valuation Technique | US Treasury and Government
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	2.96%	[1]	3.18%	[2]
Market Approach Valuation Technique | US Treasury and Government | Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	0.86%	[1]	1.07%	[2]
Market Approach Valuation Technique | US Treasury and Government | Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	4.10%	[1]	5.00%	[2]
Market Approach Valuation Technique | State and political subdivision
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	3.68%	[1]	3.88%	[2]
Market Approach Valuation Technique | State and political subdivision | Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	2.20%	[1]	2.44%	[2]
Market Approach Valuation Technique | State and political subdivision | Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	5.09%	[1]	5.79%	[2]
Market Approach Valuation Technique | Corporate
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	3.41%	[1]	3.81%	[2]
Market Approach Valuation Technique | Corporate | Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	0.86%	[1]	1.06%	[2]
Market Approach Valuation Technique | Corporate | Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	5.99%	[1]	6.75%	[2]
Market Approach Valuation Technique | Available-for-sale debt securities
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	1.96%	[1]
Market Approach Valuation Technique | Available-for-sale debt securities | Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	1.75%	[1]
Market Approach Valuation Technique | Available-for-sale debt securities | Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	3.13%	[1]
Income Approach Valuation Technique | Available-for-sale debt securities
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield			2.30%	[2]
Prepayment rate			2.00%	[2]
Default rate			2.53%	[2]
Recovery rate			10.00%	[2]
Default rate term 2			0.37%	[2]
Income Approach Valuation Technique | Available-for-sale debt securities | Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield			2.10%	[2]
Income Approach Valuation Technique | Available-for-sale debt securities | Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield			3.41%	[2]
Income Approach Valuation Technique | Fair value investments
Fair Value, Option, Quantitative Disclosures [Line Items]
Default rate	0.15%	[1]	0.25%	[2]
Recovery rate	42.00%	[1]	45.00%	[2]
Level 3 | US Treasury and Government
Fair Value, Option, Quantitative Disclosures [Line Items]
U.S. government and agency	$ 64.3	[1]	$ 58.8	[2]
Level 3 | State and political subdivision
Fair Value, Option, Quantitative Disclosures [Line Items]
State and political subdivision	48.5	[1]	45.4	[2]
Level 3 | Corporate
Fair Value, Option, Quantitative Disclosures [Line Items]
Corporate	962.4	[1]	874.1	[2]
Level 3 | Available-for-sale debt securities
Fair Value, Option, Quantitative Disclosures [Line Items]
Other ABS	10.3	[1]	11.3	[2]
Level 3 | Fair value investments
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value investments	0.9	[1]	0.8	[2]
Level 3 | EIA/VED Embedded Derivative
Fair Value, Option, Quantitative Disclosures [Line Items]
Embedded derivatives	107.4		78.9
Level 3 | GMAB/GMWB Embedded Derivatives
Fair Value, Option, Quantitative Disclosures [Line Items]
Embedded derivatives	$ (3.9)		$ (4.1)
Level 3 | Income Approach Valuation Technique | EIA/VED Embedded Derivative | Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Mortality rate	97.00%		97.00%
Lapse rate	0.02%		0.02%
Swap curve	0.18%		0.19%
Level 3 | Income Approach Valuation Technique | EIA/VED Embedded Derivative | Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Mortality rate	103.00%		103.00%
Lapse rate	47.15%		47.15%
Swap curve	3.15%		3.79%
Level 3 | Income Approach Valuation Technique | GMAB/GMWB Embedded Derivatives
Fair Value, Option, Quantitative Disclosures [Line Items]
Mortality rate	105.00%		105.00%
Level 3 | Income Approach Valuation Technique | GMAB/GMWB Embedded Derivatives | Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Lapse rate	0.00%		0.00%
Volatility surface	8.89%		10.85%
Swap curve	0.16%		0.15%
Level 3 | Income Approach Valuation Technique | GMAB/GMWB Embedded Derivatives | Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Lapse rate	40.00%		40.00%
Volatility surface	44.06%		46.33%
Swap curve	3.48%		4.15%

[1] Excludes Level 3 assets which are valued based upon non-binding independent third-party valuations or third-party price information for which unobservable inputs are not reasonably available to us.

[2] Excludes Level 3 assets which are valued based upon non-binding independent third-party valuations or third-party price information for which unobservable inputs are not reasonably available to us.