Fair Value Measurements by Input Level (Detail) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	$ 42,434.7	$ 40,035.6
Other Long-term Investments
Equity Securities	10.4	10.4
Derivative Assets	143.2	99.1
Other Liabilities
Derivative Liabilities	275.4	295.9
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	6,671.2	5,416.2
Other Long-term Investments
Equity Securities	0	0
Derivative Assets	0	0
Other Liabilities
Derivative Liabilities	0	0
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) \| United States Government and Government Agencies and Authorities
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	324.7	102.8
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) \| States, Municipalities, and Political Subdivisions
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	31.5	301.9
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) \| Foreign Governments
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	0	0.7
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) \| Public Utilities
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	1,131.4	840.1
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) \| Mortgage/Asset-backed Securities
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	0	0
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) \| All Other Corporate Bonds
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	5,183.6	4,170.7
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) \| Redeemable Preferred Stocks
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	0	0
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) \| Foreign Exchange Contract [Member]
Other Long-term Investments
Derivative Assets	0	0
Other Liabilities
Derivative Liabilities	0	0
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) \| Interest Rate Swap [Member]
Other Long-term Investments
Derivative Assets	0	0
Other Liabilities
Derivative Liabilities	0	0
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) \| Embedded Derivative Financial Instruments [Member]
Other Liabilities
Derivative Liabilities	0	0
Significant Other Observable Inputs (Level 2)
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	34,859.6	33,593.7
Other Long-term Investments
Equity Securities	0	8.9
Derivative Assets	143.2	99.1
Other Liabilities
Derivative Liabilities	154.7	199.6
Significant Other Observable Inputs (Level 2) \| United States Government and Government Agencies and Authorities
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	968.9	998.9
Significant Other Observable Inputs (Level 2) \| States, Municipalities, and Political Subdivisions
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	1,554.9	943.3
Significant Other Observable Inputs (Level 2) \| Foreign Governments
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	1,474.7	1,408.6
Significant Other Observable Inputs (Level 2) \| Public Utilities
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	8,973.5	8,670.5
Significant Other Observable Inputs (Level 2) \| Mortgage/Asset-backed Securities
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	3,127.4	3,384.8
Significant Other Observable Inputs (Level 2) \| All Other Corporate Bonds
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	18,725.4	18,154.3
Significant Other Observable Inputs (Level 2) \| Redeemable Preferred Stocks
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	34.8	33.3
Significant Other Observable Inputs (Level 2) \| Foreign Exchange Contract [Member]
Other Long-term Investments
Derivative Assets	2.4	0.7
Other Liabilities
Derivative Liabilities	121.2	160.5
Significant Other Observable Inputs (Level 2) \| Interest Rate Swap [Member]
Other Long-term Investments
Derivative Assets	140.8	98.4
Other Liabilities
Derivative Liabilities	33.5	39.1
Significant Other Observable Inputs (Level 2) \| Embedded Derivative Financial Instruments [Member]
Other Liabilities
Derivative Liabilities	0	0
Significant Unobservable Inputs (Level 3)
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	903.9	1,025.7
Other Long-term Investments
Equity Securities	10.4	1.5
Derivative Assets	0	0
Other Liabilities
Derivative Liabilities	120.7	96.3
Significant Unobservable Inputs (Level 3) \| United States Government and Government Agencies and Authorities
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	0	0
Significant Unobservable Inputs (Level 3) \| States, Municipalities, and Political Subdivisions
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	0	0
Significant Unobservable Inputs (Level 3) \| Foreign Governments
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	0	0
Significant Unobservable Inputs (Level 3) \| Public Utilities
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	395.8	173.6
Significant Unobservable Inputs (Level 3) \| Mortgage/Asset-backed Securities
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	19.6	0.7
Significant Unobservable Inputs (Level 3) \| All Other Corporate Bonds
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	466.8	829.7
Significant Unobservable Inputs (Level 3) \| Redeemable Preferred Stocks
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	21.7	21.7
Significant Unobservable Inputs (Level 3) \| Foreign Exchange Contract [Member]
Other Long-term Investments
Derivative Assets	0	0
Other Liabilities
Derivative Liabilities	0	0
Significant Unobservable Inputs (Level 3) \| Interest Rate Swap [Member]
Other Long-term Investments
Derivative Assets	0	0
Other Liabilities
Derivative Liabilities	0	0
Significant Unobservable Inputs (Level 3) \| Embedded Derivative Financial Instruments [Member]
Other Liabilities
Derivative Liabilities	120.7	96.3
United States Government and Government Agencies and Authorities
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	1,293.6	1,101.7
States, Municipalities, and Political Subdivisions
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	1,586.4	1,245.2
Foreign Governments
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	1,474.7	1,409.3
Public Utilities
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	10,500.7	9,684.2
Mortgage/Asset-backed Securities
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	3,147.0	3,385.5
All Other Corporate Bonds
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	24,375.8	23,154.7
Redeemable Preferred Stocks
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	56.5	55.0
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Available for Sale Abstract [Abstract]
Available-for-sale Securities, Fair Value Disclosure	42,434.7	40,035.6
Other Long-term Investments
Equity Securities	10.4	10.4
Derivative Assets	143.2	99.1
Other Liabilities
Derivative Liabilities	154.7	199.6
Foreign Exchange Contract [Member]
Other Long-term Investments
Derivative Assets	2.4	0.7
Other Liabilities
Derivative Liabilities	121.2	160.5
Interest Rate Swap [Member]
Other Long-term Investments
Derivative Assets	140.8	98.4
Other Liabilities
Derivative Liabilities	33.5	39.1
Embedded Derivative Financial Instruments [Member]
Other Liabilities
Derivative Liabilities	$ 120.7	$ 96.3

For an unclassified balance sheet, this item represents investments in debt and equity securities which are categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 12

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 15E

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 22

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 15D

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 13

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 15C

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 8: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 14

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 9: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 16

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 10: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6379932&loc=d3e22054-111558

+ Details

- Definition

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.

+ References+ Details

- Definition

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.

+ References+ Details

- 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.

+ References+ Details