Fair Value of Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011	Dec. 31, 2010
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 462.0	$ 820.4	$ 922.7	$ 290.4
Fixed maturities	6,794.9	6,284.7
Equity securities	392.8	246.4
Other investments	257.1	190.2
Debt	917.5	911.1
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	462.0	820.4
Fixed maturities	6,794.9	6,284.7
Equity securities	392.8	246.4
Other investments	182.5	154.4
Total financial assets	7,832.2	7,505.9
Debt	917.5	911.1
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	462.0	820.4
Fixed maturities	6,794.9	6,284.7
Equity securities	392.8	246.4
Other investments	182.8	153.9
Total financial assets	7,832.5	7,505.4
Debt	$ 1,065.3	$ 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