Fair Value of Financial Instruments - Level 3 Assets and Liabilities by Pricing Source (Details) (Level 3, USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
U.S. government and agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	$ 81.1	$ 80.2	$ 65.8	$ 58.8
State and political subdivision
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	201.2	193.3	137.8	133.8
Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	10.5	16.1	3.4	3.3
Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	1,452.5	1,417.2	1,122.3	1,091.9
Commercial mortgage-backed (“CMBS”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	6.9	64.6	6.4	31.9
Residential mortgage-backed (“RMBS”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	158.3	151.6	171.4	176.1
Collateralized debt obligations (“CDO”) / collateralized loan obligations (“CLO”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	96.5	83.5	86.2	70.9
Other asset-backed (“ABS”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	71.2	77.9	78.9	82.3
Debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	2,078.2	2,084.4	1,672.2	1,649.0
Available-for-sale equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	29.5	28.7	0	11.2
Fair value investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	33.9	33.7	35.2	35.6
Total assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	2,141.6	2,146.8	1,707.4	1,695.8
Fair Value, Measurements, Nonrecurring | Embedded Derivative Financial Instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total liabilities Internal	175.1	160.3
Total liabilities External	0	0
Total liabilities	175.1	160.3
Fair Value, Measurements, Nonrecurring | Total liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total liabilities Internal	175.1	160.3
Total liabilities External	0	0
Total liabilities	175.1	160.3
Fair Value, Measurements, Nonrecurring | U.S. government and agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	81.1	80.2
Total assets External	0	0
Total assets	81.1	80.2
Fair Value, Measurements, Nonrecurring | State and political subdivision
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	88.3	65.9
Total assets External	112.9	127.4
Total assets	201.2	193.3
Fair Value, Measurements, Nonrecurring | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	0	0
Total assets External	10.5	16.1
Total assets	10.5	16.1
Fair Value, Measurements, Nonrecurring | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	1,186.4	1,053.3
Total assets External	266.1	363.9
Total assets	1,452.5	1,417.2
Fair Value, Measurements, Nonrecurring | 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	6.9	64.6
Total assets	6.9	64.6
Fair Value, Measurements, Nonrecurring | 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	158.3	151.6
Total assets	158.3	151.6
Fair Value, Measurements, Nonrecurring | Collateralized debt obligations (“CDO”) / collateralized loan obligations (“CLO”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	0	0
Total assets External	96.5	83.5
Total assets	96.5	83.5
Fair Value, Measurements, Nonrecurring | Other asset-backed (“ABS”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	9.6	9.8
Total assets External	61.6	68.1
Total assets	71.2	77.9
Fair Value, Measurements, Nonrecurring | Debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	1,365.4	1,209.2
Total assets External	712.8	875.2
Total assets	2,078.2	2,084.4
Fair Value, Measurements, Nonrecurring | Available-for-sale equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	0	0
Total assets External	29.5	28.7
Total assets	29.5	28.7
Fair Value, Measurements, Nonrecurring | Fair value investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	1.0	1.0
Total assets External	32.9	32.7
Total assets	33.9	33.7
Fair Value, Measurements, Nonrecurring | Total assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	1,366.4	1,210.2
Total assets External	775.2	936.6
Total assets	$ 2,141.6	$ 2,146.8