Fair Value Measurements by Input Level (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fixed Maturity Securities
Fixed Maturity Securities	$ 44,436.5	$ 44,973.0
Other Long-term Investments
Derivative Assets	62.1	81.6
Equity Securities
Equity Securities	14.4	14.6
Other Liabilities
Derivative Liabilities	234.9	254.4
Interest Rate Swaps
Other Long-term Investments
Derivative Assets	59.6	76.5
Other Liabilities
Derivative Liabilities	31.2	31.7
Foreign Exchange Contracts
Other Long-term Investments
Derivative Assets	2.5	5.1
Other Liabilities
Derivative Liabilities	125.5	138.8
Credit Default Swaps
Other Liabilities
Derivative Liabilities	0.7
Embedded Derivative in Modified Coinsurance Arrangement
Other Liabilities
Derivative Liabilities	77.5	83.9
United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	1,335.1	1,348.8
States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Fixed Maturity Securities	1,853.8	1,806.8
Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	1,457.8	1,507.0
Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	11,089.2	11,144.2
Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	2,042.2	2,216.5
All Other Corporate Bonds
Fixed Maturity Securities
Fixed Maturity Securities	26,619.1	26,910.4
Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	39.3	39.3
Fair Value, Inputs, Level 1 [Member]
Fixed Maturity Securities
Fixed Maturity Securities	4,237.3	2,218.4
Other Long-term Investments
Derivative Assets	0	0
Equity Securities
Equity Securities	0	0
Other Liabilities
Derivative Liabilities	0	0
Fair Value, Inputs, Level 1 [Member] \| Interest Rate Swaps
Other Long-term Investments
Derivative Assets	0	0
Other Liabilities
Derivative Liabilities	0	0
Fair Value, Inputs, Level 1 [Member] \| Foreign Exchange Contracts
Other Long-term Investments
Derivative Assets	0	0
Other Liabilities
Derivative Liabilities	0	0
Fair Value, Inputs, Level 1 [Member] \| Credit Default Swaps
Other Liabilities
Derivative Liabilities	0
Fair Value, Inputs, Level 1 [Member] \| Embedded Derivative in Modified Coinsurance Arrangement
Other Liabilities
Derivative Liabilities	0	0
Fair Value, Inputs, Level 1 [Member] \| United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	359.5	104.1
Fair Value, Inputs, Level 1 [Member] \| States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Fixed Maturity Securities	33.5	53.0
Fair Value, Inputs, Level 1 [Member] \| Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	0	0
Fair Value, Inputs, Level 1 [Member] \| Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	596.3	84.2
Fair Value, Inputs, Level 1 [Member] \| Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	15.5	0
Fair Value, Inputs, Level 1 [Member] \| All Other Corporate Bonds
Fixed Maturity Securities
Fixed Maturity Securities	3,232.5	1,977.1
Fair Value, Inputs, Level 1 [Member] \| Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	0	0
Fair Value, Inputs, Level 2 [Member]
Fixed Maturity Securities
Fixed Maturity Securities	38,521.7	40,766.3
Other Long-term Investments
Derivative Assets	62.1	81.6
Equity Securities
Equity Securities	10.1	10.3
Other Liabilities
Derivative Liabilities	157.4	170.5
Fair Value, Inputs, Level 2 [Member] \| Interest Rate Swaps
Other Long-term Investments
Derivative Assets	59.6	76.5
Other Liabilities
Derivative Liabilities	31.2	31.7
Fair Value, Inputs, Level 2 [Member] \| Foreign Exchange Contracts
Other Long-term Investments
Derivative Assets	2.5	5.1
Other Liabilities
Derivative Liabilities	125.5	138.8
Fair Value, Inputs, Level 2 [Member] \| Credit Default Swaps
Other Liabilities
Derivative Liabilities	0.7
Fair Value, Inputs, Level 2 [Member] \| Embedded Derivative in Modified Coinsurance Arrangement
Other Liabilities
Derivative Liabilities	0	0
Fair Value, Inputs, Level 2 [Member] \| United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	975.6	1,244.7
Fair Value, Inputs, Level 2 [Member] \| States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Fixed Maturity Securities	1,645.5	1,625.1
Fair Value, Inputs, Level 2 [Member] \| Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	1,375.2	1,424.9
Fair Value, Inputs, Level 2 [Member] \| Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	10,090.4	10,485.6
Fair Value, Inputs, Level 2 [Member] \| Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	2,026.2	2,216.0
Fair Value, Inputs, Level 2 [Member] \| All Other Corporate Bonds
Fixed Maturity Securities
Fixed Maturity Securities	22,394.3	23,755.5
Fair Value, Inputs, Level 2 [Member] \| Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	14.5	14.5
Fair Value, Inputs, Level 3 [Member]
Fixed Maturity Securities
Fixed Maturity Securities	1,677.5	1,988.3
Other Long-term Investments
Derivative Assets	0	0
Equity Securities
Equity Securities	4.3	4.3
Other Liabilities
Derivative Liabilities	77.5	83.9
Fair Value, Inputs, Level 3 [Member] \| Interest Rate Swaps
Other Long-term Investments
Derivative Assets	0	0
Other Liabilities
Derivative Liabilities	0	0
Fair Value, Inputs, Level 3 [Member] \| Foreign Exchange Contracts
Other Long-term Investments
Derivative Assets	0	0
Other Liabilities
Derivative Liabilities	0	0
Fair Value, Inputs, Level 3 [Member] \| Credit Default Swaps
Other Liabilities
Derivative Liabilities	0
Fair Value, Inputs, Level 3 [Member] \| Embedded Derivative in Modified Coinsurance Arrangement
Other Liabilities
Derivative Liabilities	77.5	83.9
Fair Value, Inputs, Level 3 [Member] \| United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	0	0
Fair Value, Inputs, Level 3 [Member] \| States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Fixed Maturity Securities	174.8	128.7
Fair Value, Inputs, Level 3 [Member] \| Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	82.6	82.1
Fair Value, Inputs, Level 3 [Member] \| Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	402.5	574.4
Fair Value, Inputs, Level 3 [Member] \| Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	0.5	0.5
Fair Value, Inputs, Level 3 [Member] \| All Other Corporate Bonds
Fixed Maturity Securities
Fixed Maturity Securities	992.3	1,177.8
Fair Value, Inputs, Level 3 [Member] \| Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	$ 24.8	$ 24.8

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

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Fair values as of the balance sheet date of all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

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- Definition

Fair values as of the balance sheet date of all liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

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