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, 2014	Jun. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012
Embedded Derivative Financial Instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total liabilities Internal	$ 116.4		$ 74.8
Total liabilities External	0		0
Total liabilities	116.4		74.8
Total liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total liabilities Internal	116.4		74.8
Total liabilities External	0		0
Total liabilities	116.4		74.8
U.S. government and agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	62.0		58.8
Total assets External	4.0		0
Total assets	66.0	64.3	58.8	59.1	47.1	25.9
State and political subdivision
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	50.1		45.4
Total assets External	111.3		88.4
Total assets	161.4	144.6	133.8	124.4	104.3	116.1
Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	0		0
Total assets External	3.4		3.3
Total assets	3.4	3.5	3.3	3.3	3.3	8.0
Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	954.9		874.1
Total assets External	272.9		228.2
Total assets	1,227.8	1,145.1	1,102.3	1,015.2	955.9	801.5
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	2.7		31.9
Total assets	2.7	2.9	31.9	41.2	33.5	19.3
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	156.1		175.9
Total assets	156.1	161.0	175.9	190.4	202.5	222.6
Collateralized debt obligation (“CDO”) / collateralized loan obligation (“CLO”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	0		0
Total assets External	92.9		70.9
Total assets	92.9	96.9	70.9	66.7	67.6	56.8
Other asset-backed (“ABS”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	10.1		11.3
Total assets External	61.5		71.0
Total assets	71.6	74.7	82.3	81.8	73.9	80.1
Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	1,077.1		989.6
Total assets External	704.8		669.6
Total assets	1,781.9	1,693.0	1,659.2	1,582.1	1,488.1	1,330.3
Fair value investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	1.0		0.8
Total assets External	33.2		34.8
Total assets	34.2	34.6	35.6	39.4	34.4	22.7
Total assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	1,078.1		990.4
Total assets External	738.0		704.4
Total assets	$ 1,816.1	$ 1,727.6	$ 1,694.8	$ 1,621.5	$ 1,522.5	$ 1,353.0