Fair Value of Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 564.8	$ 820.4	$ 290.4	$ 316.5
Fixed maturities	6,952.2	6,284.7
Equity securities	315.8	246.4
Other investments	210.3	190.2
Debt	849.4	911.1
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	564.8	820.4
Fixed maturities	6,952.2	6,284.7
Equity securities	315.8	246.4
Other investments	188.9	154.4
Total financial assets	8,021.7	7,505.9
Debt	849.4	911.1
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	564.8	820.4
Fixed maturities	6,952.2	6,284.7
Equity securities	315.8	246.4
Other investments	189.4	153.9
Total financial assets	8,022.2	7,505.4
Debt	$ 995.2	$ 1,014.9

Amount of debt securities categorized neither as held-to-maturity nor trading.

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+ Details

- Definition

Amount of equity securities categorized neither as held-to-maturity nor as trading.

+ References

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+ Details

- Definition

Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

+ References

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+ Details

- Definition

Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References+ Details