Summary Of Investments - Other Than Investments In Related Parties (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 6,435.9	[1]
Value
Amount at which shown in the balance sheet	6,721.3
Fixed Maturities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	5,736.7	[1]
Value	6,012.8
Amount at which shown in the balance sheet	6,012.8
United States Government And Government Agencies And Authorities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	827.8	[1]
Value	867.7
Amount at which shown in the balance sheet	867.7
States and Political Subdivisions [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	963.6	[1]
Value	1,027.1
Amount at which shown in the balance sheet	1,027.1
Foreign Government [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	126.3	[1]
Value	126.3
Amount at which shown in the balance sheet	126.3
Public Utilities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	507.1	[1]
Value	546.7
Amount at which shown in the balance sheet	546.7
All Other Corporate Bonds [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	3,311.9	[1]
Value	3,445.0
Amount at which shown in the balance sheet	3,445.0
Total Equity Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	239.9	[1]
Value	246.4
Amount at which shown in the balance sheet	246.4
Public Utility, Equity Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	59.6	[1]
Value	65.1
Amount at which shown in the balance sheet	65.1
Banks, Trust And Insurance Companies [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	14.9	[1]
Value	14.5
Amount at which shown in the balance sheet	14.5
Industrial, Miscellaneous And All Other [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	132.8	[1]
Value	132.6
Amount at which shown in the balance sheet	132.6
Nonredeemable Preferred Stock [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	32.6	[1]
Value	34.2
Amount at which shown in the balance sheet	34.2
Mortgage Loans [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	4.7	[1]
Value
Amount at which shown in the balance sheet	4.7
Real Estate Investment [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	22.8	[1]
Value
Amount at which shown in the balance sheet	22.8
Other Long-term Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	159.8	[1],[2]
Value		[2]
Amount at which shown in the balance sheet	162.7	[2]
Short-Term Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	272.0	[1]
Value
Amount at which shown in the balance sheet	$ 271.9

[1] The cost of other long-term investments differs from the carrying value due to market value changes in the Company's equity ownership of limited partnership investments.

[2] For equity securities, represents original cost, and for fixed maturities, original cost reduced by repayments and adjusted for amortization of premiums and accretion of discounts.