Fair Value of Financial Instruments - Level 3 Assets and Liabilities by Pricing Source (Details) (Level 3, Fair Value, Measurements, Nonrecurring, USD $) In Millions, unless otherwise specified	Mar. 31, 2014 Embedded derivatives		Dec. 31, 2013 Embedded derivatives		Mar. 31, 2014 Total liabilities		Dec. 31, 2013 Total liabilities		Mar. 31, 2014 U.S. government and agency		Dec. 31, 2013 U.S. government and agency		Mar. 31, 2013 U.S. government and agency		Dec. 31, 2012 U.S. government and agency		Mar. 31, 2014 State and political subdivision		Dec. 31, 2013 State and political subdivision		Mar. 31, 2013 State and political subdivision	Dec. 31, 2011 State and political subdivision	Mar. 31, 2014 Foreign government		Dec. 31, 2013 Foreign government		Mar. 31, 2013 Foreign government	Dec. 31, 2011 Foreign government	Mar. 31, 2014 Corporate		Dec. 31, 2013 Corporate		Mar. 31, 2013 Corporate	Dec. 31, 2011 Corporate	Mar. 31, 2014 Commercial mortgage-backed (“CMBS”)		Dec. 31, 2013 Commercial mortgage-backed (“CMBS”)		Mar. 31, 2013 Commercial mortgage-backed (“CMBS”)	Dec. 31, 2011 Commercial mortgage-backed (“CMBS”)	Mar. 31, 2014 Residential mortgage-backed (“RMBS”)		Dec. 31, 2013 Residential mortgage-backed (“RMBS”)		Mar. 31, 2013 Residential mortgage-backed (“RMBS”)	Dec. 31, 2011 Residential mortgage-backed (“RMBS”)	Mar. 31, 2014 Collateralized debt obligation (“CDO”) / collateralized loan obligation (“CLO”)		Dec. 31, 2013 Collateralized debt obligation (“CDO”) / collateralized loan obligation (“CLO”)		Mar. 31, 2013 Collateralized debt obligation (“CDO”) / collateralized loan obligation (“CLO”)	Dec. 31, 2011 Collateralized debt obligation (“CDO”) / collateralized loan obligation (“CLO”)	Mar. 31, 2014 Other asset-backed		Dec. 31, 2013 Other asset-backed		Mar. 31, 2013 Other asset-backed	Dec. 31, 2011 Other asset-backed	Mar. 31, 2014 Debt Securities		Dec. 31, 2013 Debt Securities		Mar. 31, 2013 Debt Securities	Dec. 31, 2011 Debt Securities	Mar. 31, 2014 Fair value investments		Dec. 31, 2013 Fair value investments		Mar. 31, 2013 Fair value investments	Dec. 31, 2011 Fair value investments	Mar. 31, 2014 Total assets		Dec. 31, 2013 Total assets		Mar. 31, 2013 Total assets	Dec. 31, 2011 Total assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal									$ 65.8	[1],[2]	$ 58.8	[1],[2]					$ 46.4	[1]	$ 45.4	[1]			$ 0	[1]	$ 0	[1]			$ 948.7	[1]	$ 874.1	[1]			$ 0	[1]	$ 0	[1]			$ 0	[1]	$ 0	[1]			$ 0	[1]	$ 0	[1]			$ 10.4	[1]	$ 11.3	[1]			$ 1,071.3	[1]	$ 989.6	[1]			$ 0.8	[1]	$ 0.8	[1]			$ 1,072.1	[1]	$ 990.4	[1]
Total assets External									0	[2],[3]	0	[2],[3]					91.4	[3]	88.4	[3]			3.4	[3]	3.3	[3]			173.6	[3]	228.2	[3]			6.4	[3]	31.9	[3]			170.9	[3]	175.9	[3]			86.2	[3]	70.9	[3]			68.5	[3]	71.0	[3]			600.4	[3]	669.6	[3]			34.4	[3]	34.8	[3]			634.8	[3]	704.4	[3]
Total assets									65.8	[2]	58.8	[2]	31.4	[2]	25.9	[2]	137.8		133.8		98.2	116.1	3.4		3.3		9.5	8.0	1,122.3		1,102.3		903.2	801.5	6.4		31.9		18.7	19.3	170.9		175.9		214.3	222.6	86.2		70.9		62.2	56.8	78.9		82.3		79.9	80.1	1,671.7		1,659.2		1,417.4	1,330.3	35.2		35.6		31.4	22.7	1,706.9		1,694.8		1,448.8	1,353.0
Total liabilities Internal	93.7	[1]	74.8	[1]	93.7	[1]	74.8	[1]
Total liabilities External	0	[3]	0	[3]	0	[3]	0	[3]
Total liabilities	$ 93.7		$ 74.8		$ 93.7		$ 74.8

[1] Represents valuations reflecting both internally-derived and market inputs, as well as third-party information or quotes.

[2] Includes securities whose underlying collateral is an obligation of a U.S. government entity.

[3] Represents unadjusted prices from independent pricing services, third-party financial statements and independent indicative broker quotes where pricing inputs are not readily available.