Fair Value of Financial Instruments - Quantitative Estimates for Level 3 Assets (Details) (USD $) In Millions, unless otherwise specified																	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended		6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2013 US Treasury and Government Level 3	Dec. 31, 2012 US Treasury and Government Level 3	Mar. 31, 2013 State and political subdivision Level 3	Dec. 31, 2012 State and political subdivision Level 3	Mar. 31, 2013 Corporate Level 3	Dec. 31, 2012 Corporate Level 3	Mar. 31, 2013 CDO/CLO Level 3	Dec. 31, 2012 CDO/CLO Level 3	Mar. 31, 2013 Other asset-backed Level 3	Dec. 31, 2012 Other asset-backed Level 3	Mar. 31, 2013 Fair value investments Level 3	Dec. 31, 2012 Fair value investments Level 3	Mar. 31, 2013 EIA/VED Embedded Derivative Level 3	Dec. 31, 2012 EIA/VED Embedded Derivative Level 3	Mar. 31, 2013 GMAB/GMWB Embedded Derivatives Level 3	Dec. 31, 2012 GMAB/GMWB Embedded Derivatives Level 3	Jun. 30, 2013 Market Approach Valuation Technique US Treasury and Government	Dec. 31, 2012 Market Approach Valuation Technique US Treasury and Government	Jun. 30, 2013 Market Approach Valuation Technique US Treasury and Government Minimum	Dec. 31, 2012 Market Approach Valuation Technique US Treasury and Government Minimum	Jun. 30, 2013 Market Approach Valuation Technique US Treasury and Government Maximum	Dec. 31, 2012 Market Approach Valuation Technique US Treasury and Government Maximum	Jun. 30, 2013 Market Approach Valuation Technique State and political subdivision	Dec. 31, 2012 Market Approach Valuation Technique State and political subdivision	Jun. 30, 2013 Market Approach Valuation Technique State and political subdivision Minimum	Dec. 31, 2012 Market Approach Valuation Technique State and political subdivision Minimum	Jun. 30, 2013 Market Approach Valuation Technique State and political subdivision Maximum	Dec. 31, 2012 Market Approach Valuation Technique State and political subdivision Maximum	Jun. 30, 2013 Market Approach Valuation Technique Corporate	Dec. 31, 2012 Market Approach Valuation Technique Corporate	Jun. 30, 2013 Market Approach Valuation Technique Corporate Minimum	Dec. 31, 2012 Market Approach Valuation Technique Corporate Minimum	Jun. 30, 2013 Market Approach Valuation Technique Corporate Maximum	Dec. 31, 2012 Market Approach Valuation Technique Corporate Maximum	Jun. 30, 2013 Income Approach Valuation Technique CDO/CLO	Dec. 31, 2012 Income Approach Valuation Technique CDO/CLO	Jun. 30, 2013 Income Approach Valuation Technique Collateralized Loan Obligations	Dec. 31, 2012 Income Approach Valuation Technique Collateralized Loan Obligations	Jun. 30, 2013 Income Approach Valuation Technique High Yield Bonds	Dec. 31, 2012 Income Approach Valuation Technique High Yield Bonds	Jun. 30, 2013 Income Approach Valuation Technique Investment Grade Bonds	Dec. 31, 2012 Income Approach Valuation Technique Investment Grade Bonds	Jun. 30, 2013 Income Approach Valuation Technique Other asset-backed	Dec. 31, 2012 Income Approach Valuation Technique Other asset-backed	Jun. 30, 2013 Income Approach Valuation Technique Other asset-backed Minimum	Dec. 31, 2012 Income Approach Valuation Technique Other asset-backed Minimum	Dec. 31, 2012 Income Approach Valuation Technique Other asset-backed Maximum	Jun. 30, 2013 Income Approach Valuation Technique Fair value investments	Dec. 31, 2012 Income Approach Valuation Technique Fair value investments	Jun. 30, 2013 Income Approach Valuation Technique EIA/VED Embedded Derivative Level 3	Dec. 31, 2012 Income Approach Valuation Technique EIA/VED Embedded Derivative Level 3	Jun. 30, 2013 Income Approach Valuation Technique EIA/VED Embedded Derivative Minimum Level 3	Dec. 31, 2012 Income Approach Valuation Technique EIA/VED Embedded Derivative Minimum Level 3	Jun. 30, 2013 Income Approach Valuation Technique EIA/VED Embedded Derivative Maximum Level 3	Dec. 31, 2012 Income Approach Valuation Technique EIA/VED Embedded Derivative Maximum Level 3	Jun. 30, 2013 Income Approach Valuation Technique GMAB/GMWB Embedded Derivatives Level 3	Dec. 31, 2012 Income Approach Valuation Technique GMAB/GMWB Embedded Derivatives Level 3	Jun. 30, 2013 Income Approach Valuation Technique GMAB/GMWB Embedded Derivatives Minimum Level 3	Dec. 31, 2012 Income Approach Valuation Technique GMAB/GMWB Embedded Derivatives Minimum Level 3	Jun. 30, 2013 Income Approach Valuation Technique GMAB/GMWB Embedded Derivatives Maximum Level 3	Dec. 31, 2012 Income Approach Valuation Technique GMAB/GMWB Embedded Derivatives Maximum Level 3
Fair Value, Option, Quantitative Disclosures [Line Items]
U.S. government and agency	$ 47.1	$ 25.9
State and political subdivision			38.2	58.0
Corporate					762.4	649.6
CDO/CLO							7.0	3.4
Other asset-backed									2.6	2.8
Fair value investments											0.7	0.8
Yield																	2.71%	2.57%	1.37%	1.46%	4.01%	3.29%	3.51%	2.94%	2.55%	1.94%	4.17%	3.53%	3.70%	3.01%	1.26%	1.47%	6.69%	6.33%										4.97%	3.26%	2.61%	9.50%
Prepayment rate																																					20.00%	20.00%					2.00%	2.00%
Default rate																																					2.55%	2.50%					2.53%	2.53%				0.45%	0.24%
Recovery rate																																					65.00%	65.00%	35.00%	35.00%	45.00%	45.00%	10.00%	10.00%				45.00%	45.00%
Reinvestment spread																																			4.00%	4.00%
Default rate term 2																																											0.37%	0.33%
Embedded derivatives													$ 62.8	$ 51.2	$ 14.1	$ 28.3
Mortality rate																																																		75.00%	75.00%					75.00%	75.00%
Lapse rate																																																				0.60%	1.00%	35.00%	35.00%			0.00%	0.00%	60.00%	60.00%
Volatility surface																																																										10.71%	11.67%	47.47%	50.83%
Swap curve																																																				0.24%	0.21%	3.32%	2.50%			0.16%	0.36%	3.61%	3.17%