Schedule I - Summary of Investments - Other Than Investments in Related Parties (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	$ 10,261.2	[1]
Amount at which shown in balance sheet	10,868.5
Short-term Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	1,124.8	[1]
Amount at which shown in balance sheet	1,124.8
Miscellaneous investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	21.6	[1]
Amount at which shown in balance sheet	21.6
Other investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	5.3	[1]
Amount at which shown in balance sheet	5.3
Total Investments, Excluding Other Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	10,255.9	[1]
Amount at which shown in balance sheet	10,863.1
Fixed maturity securities [Member] | United States Government and government agencies and authorities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	1,004.6	[1]
Fair Value	1,028.6
Amount at which shown in balance sheet	1,028.6
Fixed maturity securities [Member] | States, municipalities and political subdivisions [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	168.1	[1]
Fair Value	171.3
Amount at which shown in balance sheet	171.3
Fixed maturity securities [Member] | Foreign government [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	128.4	[1]
Fair Value	132.5
Amount at which shown in balance sheet	132.5
Fixed maturity securities [Member] | Corporate, industrial and all other [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	7,176.0	[1]
Fair Value	7,379.8
Amount at which shown in balance sheet	7,379.8
Fixed maturity securities [Member] | Fixed Maturities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	8,477.3	[1]
Fair Value	8,712.3
Amount at which shown in balance sheet	8,712.3
Equity Securities [Member] | Non-redeemable preferred stock [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	0.6	[1]
Fair Value	0.7
Amount at which shown in balance sheet	0.7
Equity Securities [Member] | Banks, trust and insurance companies [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	31.8	[1]
Fair Value	223.5
Amount at which shown in balance sheet	223.5
Equity Securities [Member] | Industrial, miscellaneous, and all others [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	435.9	[1]
Fair Value	540.1
Amount at which shown in balance sheet	540.1
Equity Securities [Member] | Indexed mutual funds [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	163.6	[1]
Fair Value	239.8
Amount at which shown in balance sheet	239.8
Equity Securities [Member] | Equity Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	632.0	[1]
Fair Value	1,004.2
Amount at which shown in balance sheet	$ 1,004.2

[1] (1)Represents original cost of equity securities, and as to fixed maturities, original cost reduced by repayments and adjusted for amortization of premium or accrual of discount.