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	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011
Embedded Derivative Financial Instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total liabilities Internal	$ 72.6		$ 79.5
Total liabilities External	0		0
Total liabilities	72.6		79.5
Total liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total liabilities Internal	72.6		79.5
Total liabilities External	0		0
Total liabilities	72.6		79.5
U.S. government and agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	59.1		25.9
Total assets External	0		0
Total assets	59.1	47.1	25.9	28.6	27.7	31.7
State and political subdivision
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	45.5		58.0
Total assets External	78.9		58.1
Total assets	124.4	104.3	116.1	93.4	90.1	48.7
Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	0		0
Total assets External	3.3		8.0
Total assets	3.3	3.3	8.0	7.8	7.2	3.1
Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	801.0		649.6
Total assets External	214.2		151.9
Total assets	1,015.2	955.9	801.5	732.7	664.5	513.8
Commercial mortgage-backed (“CMBS”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	0		0
Total assets External	41.2		19.3
Total assets	41.2	33.5	19.3	18.0	17.8	35.4
Residential mortgage-backed (“RMBS”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	0		0
Total assets External	190.4		222.6
Total assets	190.4	202.5	222.6	245.6	247.3	268.0
CDO/CLO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	0		3.4
Total assets External	66.7		53.4
Total assets	66.7	67.6	56.8	58.9	57.3	62.5
Other asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	11.5		2.8
Total assets External	70.3		77.3
Total assets	81.8	73.9	80.1	83.4	71.1	81.2
Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	917.1		739.7
Total assets External	665.0		590.6
Total assets	1,582.1	1,488.1	1,330.3	1,268.4	1,183.0	1,044.4
Related party reinsurance derivative
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	0		0
Total assets External	0		0
Total assets	0	0	0	0	4.2	3.5
Fair value investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	0.8		0.8
Total assets External	38.6		21.9
Total assets	39.4	34.4	22.7	28.3	28.9	27.5
Total assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	917.9		740.5
Total assets External	703.6		612.5
Total assets	$ 1,621.5	$ 1,522.5	$ 1,353.0	$ 1,296.7	$ 1,216.1	$ 1,075.4