Schedule 1 - Investments Other Than Investments in Related Parties (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	$ 5,933.1
Amount Carried in Balance Sheet	6,439.7
Fixed Maturities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	4,341.7
Fair Value	4,777.6
Amount Carried in Balance Sheet	4,777.6
US Government and Government Agencies and Authorities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	315.2
Fair Value	345.5
Amount Carried in Balance Sheet	345.5
States and Political Subdivisions [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	1,352.5
Fair Value	1,477.1
Amount Carried in Balance Sheet	1,477.1
Other Corporate Bonds and Notes [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	2,599.3
Fair Value	2,878.5
Amount Carried in Balance Sheet	2,878.5
Redeemable Preferred Stocks [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	5.9
Fair Value	6.7
Amount Carried in Balance Sheet	6.7
Collateralized Loan Obligations [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	64.9
Fair Value	64.4
Amount Carried in Balance Sheet	64.4
Other Mortgage and Asset-backed Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	3.9
Fair Value	5.4
Amount Carried in Balance Sheet	5.4
Equity Securities, Investment Summary [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	561.5
Fair Value	632.2
Amount Carried in Balance Sheet	632.2
Preferred Stock [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	101.8
Fair Value	109.5
Amount Carried in Balance Sheet	109.5
Common Stock [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	104.2
Fair Value	133.8
Amount Carried in Balance Sheet	133.8
Other Equity Interests [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	355.5
Fair Value	388.9
Amount Carried in Balance Sheet	388.9
Fair Value Option Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	53.3
Fair Value	53.3
Amount Carried in Balance Sheet	53.3
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	184.8
Amount Carried in Balance Sheet	184.8
Loans, Real Estate and Other Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	449.6
Amount Carried in Balance Sheet	449.6
Short-term Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	342.2
Amount Carried in Balance Sheet	$ 342.2