Schedule 1 - Investments Other Than Investments in Related Parties (Details) $ in Millions	Dec. 31, 2015 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	$ 6,101.6
Amount Carried in Balance Sheet	6,429.5
Fixed Maturities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	4,560.7
Fair Value	4,852.3
Amount Carried in Balance Sheet	4,852.3
US Government and Government Agencies and Authorities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	298.0
Fair Value	320.6
Amount Carried in Balance Sheet	320.6
States and Political Subdivisions [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	1,513.7
Fair Value	1,622.6
Amount Carried in Balance Sheet	1,622.6
Other Corporate Bonds and Notes [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	2,651.5
Fair Value	2,812.8
Amount Carried in Balance Sheet	2,812.8
Redeemable Preferred Stocks [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	3.7
Fair Value	3.8
Amount Carried in Balance Sheet	3.8
Collateralized Loan Obligations [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	90.0
Fair Value	87.3
Amount Carried in Balance Sheet	87.3
Other Mortgage and Asset-backed Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	3.8
Fair Value	5.2
Amount Carried in Balance Sheet	5.2
Equity Securities, Investment Summary [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	486.9
Fair Value	523.2
Amount Carried in Balance Sheet	523.2
Preferred Stock [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	97.9
Fair Value	103.9
Amount Carried in Balance Sheet	103.9
Common Stock [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	28.3
Fair Value	36.7
Amount Carried in Balance Sheet	36.7
Other Equity Interests [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	360.7
Fair Value	382.6
Amount Carried in Balance Sheet	382.6
Fair Value Option Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	164.5
Fair Value	164.5
Amount Carried in Balance Sheet	164.5
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	190.6
Amount Carried in Balance Sheet	190.6
Loans, Real Estate and Other Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	443.2
Amount Carried in Balance Sheet	443.2
Short-term Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	255.7
Amount Carried in Balance Sheet	$ 255.7