Schedule 1 - Investments Other Than Investments in Related Parties (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	$ 5,824.5
Amount Carried in Balance Sheet	6,460.1
Fixed Maturities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	4,283.8
Fair Value	4,860.2
Amount Carried in Balance Sheet	4,860.2
US Government and Government Agencies and Authorities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	384.0
Fair Value	428.9
Amount Carried in Balance Sheet	428.9
States and Political Subdivisions [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	1,251.0
Fair Value	1,401.4
Amount Carried in Balance Sheet	1,401.4
Other Corporate Bonds and Notes [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	2,618.7
Fair Value	2,997.3
Amount Carried in Balance Sheet	2,997.3
Redeemable Preferred Stocks [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	30.1
Fair Value	32.6
Amount Carried in Balance Sheet	32.6
Equity Securities, Investment Summary [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	462.7
Fair Value	521.9
Amount Carried in Balance Sheet	521.9
Preferred Stock [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	93.8
Fair Value	100.5
Amount Carried in Balance Sheet	100.5
Common Stock [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	126.1
Fair Value	154.2
Amount Carried in Balance Sheet	154.2
Other Equity Interests [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	242.8
Fair Value	267.2
Amount Carried in Balance Sheet	267.2
Equity Method Limited Liability Investments at Cost Plus Cumulative Undistributed Earnings [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	253.0
Amount Carried in Balance Sheet	253.0
Loans, Real Estate and Other Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	497.5
Amount Carried in Balance Sheet	497.5
Short-term Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	327.5
Amount Carried in Balance Sheet	$ 327.5