Fair Value of Financial Instruments - Level 3 Assets and Liabilities by Pricing Source (Details) - Level 3 - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	$ 2,175.1	$ 2,146.8
Fair value, liabilities	175.4	160.3
Embedded derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, liabilities	175.4	160.3
U.S. government and agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	86.1	80.2
State and political subdivision
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	203.6	193.3
Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	10.7	16.1
Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	1,463.3	1,417.2
Commercial mortgage-backed (“CMBS”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	8.9	64.6
Residential mortgage-backed (“RMBS”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	135.4	151.6
Collateralized debt obligations (“CDO”) / collateralized loan obligations (“CLO”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	139.4	83.5
Other asset-backed (“ABS”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	70.7	77.9
Debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	2,118.1	2,084.4
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	27.6	28.7
Fair value investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	29.4	33.7
Internal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	1,351.9	1,210.2
Fair value, liabilities	175.4	160.3
Internal | Embedded derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, liabilities	175.4	160.3
Internal | U.S. government and agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	86.1	80.2
Internal | State and political subdivision
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	94.6	65.9
Internal | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	0.0	0.0
Internal | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	1,160.9	1,053.3
Internal | Commercial mortgage-backed (“CMBS”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	0.0	0.0
Internal | Residential mortgage-backed (“RMBS”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	0.0	0.0
Internal | Collateralized debt obligations (“CDO”) / collateralized loan obligations (“CLO”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	0.0	0.0
Internal | Other asset-backed (“ABS”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	9.3	9.8
Internal | Debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	1,350.9	1,209.2
Internal | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	0.0	0.0
Internal | Fair value investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	1.0	1.0
External
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	823.2	936.6
Fair value, liabilities	0.0	0.0
External | Embedded derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, liabilities	0.0	0.0
External | U.S. government and agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	0.0	0.0
External | State and political subdivision
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	109.0	127.4
External | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	10.7	16.1
External | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	302.4	363.9
External | Commercial mortgage-backed (“CMBS”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	8.9	64.6
External | Residential mortgage-backed (“RMBS”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	135.4	151.6
External | Collateralized debt obligations (“CDO”) / collateralized loan obligations (“CLO”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	139.4	83.5
External | Other asset-backed (“ABS”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	61.4	68.1
External | Debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	767.2	875.2
External | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	27.6	28.7
External | Fair value investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets	$ 28.4	$ 32.7