Fair Value (Fair Value of Financial Instruments) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 606.2	$ 486.2	$ 690.4	$ 564.8
Fixed maturities	7,058.5	6,970.6
Equity securities	530.5	430.2
Other investments	238.5	192.5
Debt	903.9	903.9
Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	606.2	486.2
Fixed maturities	7,058.5	6,970.6
Equity securities	530.5	430.2
Other investments	218.4	173.1
Total financial assets	8,413.6	8,060.1
Debt	903.9	903.9
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	606.2	486.2
Fixed maturities	7,058.5	6,970.6
Equity securities	530.5	430.2
Other investments	220.9	173.7
Total financial assets	8,416.1	8,060.7
Debt	$ 1,030.7	$ 961.7

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.

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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.

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