Summary of Investments Other than Investments in Related Parties (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 7,035.6	[1]
Amount at which shown in the balance sheet	7,478.3
United States Government and agencies and authorities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	900.1	[1]
Value	935.7
Amount at which shown in the balance sheet	935.7
States, municipalities and political subdivisions
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,010.2	[1]
Value	1,096.3
Amount at which shown in the balance sheet	1,096.3
Foreign governments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	345.3	[1]
Value	349.7
Amount at which shown in the balance sheet	349.7
Public utilities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	515.2	[1]
Value	563.7
Amount at which shown in the balance sheet	563.7
All other corporate bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	3,751.4	[1]
Value	3,999.5
Amount at which shown in the balance sheet	3,999.5
Fixed Maturities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	6,522.2	[1]
Value	6,944.9
Amount at which shown in the balance sheet	6,944.9
Public utilities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	64.9	[1]
Value	62.8
Amount at which shown in the balance sheet	62.8
Banks, trust and insurance companies
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	10.7	[1]
Value	10.8
Amount at which shown in the balance sheet	10.8
Industrial, miscellaneous and all other
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	189.7	[1]
Value	201.6
Amount at which shown in the balance sheet	201.6
Nonredeemable preferred stocks
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	33.7	[1]
Value	40.6
Amount at which shown in the balance sheet	40.6
Equity securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	299.0	[1]
Value	315.8
Amount at which shown in the balance sheet	315.8
Mortgage loans on real estate
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	3.9	[1]
Amount at which shown in the balance sheet	3.9
Real estate
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	6.7	[1]
Amount at which shown in the balance sheet	6.7
Other Long-term Investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	196.5	[1],[2]
Amount at which shown in the balance sheet	199.7	[2]
Short-term Investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	7.3	[1]
Amount at which shown in the balance sheet	$ 7.3

[1] 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.

[2] 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.