Fair Value Measurements by Input Level (Detail) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	$ 40,768.4	$ 40,035.6
Derivatives
Interest Rate Swaps	84.8	98.4
Foreign Exchange Contracts	0.2	0.7
Total Derivatives	85.0	99.1
Equity Securities	13.2	10.4
Derivatives
Interest Rate Swaps	33.2	39.1
Foreign Exchange Contracts	170.4	160.5
Embedded Derivative in Modified Coinsurance Arrangement	87.0	96.3
Total Derivatives	290.6	295.9
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	6,862.1	5,416.2
Derivatives
Interest Rate Swaps	0	0
Foreign Exchange Contracts	0	0
Total Derivatives	0	0
Equity Securities	9.6	0
Derivatives
Interest Rate Swaps	0	0
Foreign Exchange Contracts	0	0
Embedded Derivative in Modified Coinsurance Arrangement	0	0
Total Derivatives	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	120.9	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	49.0	301.9
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) \| Foreign Governments
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	7.6	0.7
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) \| Public Utilities
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	1,074.4	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	5,610.2	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
Significant Other Observable Inputs (Level 2)
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	33,117.8	33,593.7
Derivatives
Interest Rate Swaps	84.8	98.4
Foreign Exchange Contracts	0.2	0.7
Total Derivatives	85.0	99.1
Equity Securities	0	8.9
Derivatives
Interest Rate Swaps	33.2	39.1
Foreign Exchange Contracts	170.4	160.5
Embedded Derivative in Modified Coinsurance Arrangement	0	0
Total Derivatives	203.6	199.6
Significant Other Observable Inputs (Level 2) \| United States Government and Government Agencies and Authorities
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	1,009.6	998.9
Significant Other Observable Inputs (Level 2) \| States, Municipalities, and Political Subdivisions
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	1,320.1	943.3
Significant Other Observable Inputs (Level 2) \| Foreign Governments
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	1,442.6	1,408.6
Significant Other Observable Inputs (Level 2) \| Public Utilities
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	8,594.1	8,670.5
Significant Other Observable Inputs (Level 2) \| Mortgage/Asset-backed Securities
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	3,219.5	3,384.8
Significant Other Observable Inputs (Level 2) \| All Other Corporate Bonds
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	17,497.0	18,154.3
Significant Other Observable Inputs (Level 2) \| Redeemable Preferred Stocks
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	34.9	33.3
Significant Unobservable Inputs (Level 3)
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	788.5	1,025.7
Derivatives
Interest Rate Swaps	0	0
Foreign Exchange Contracts	0	0
Total Derivatives	0	0
Equity Securities	3.6	1.5
Derivatives
Interest Rate Swaps	0	0
Foreign Exchange Contracts	0	0
Embedded Derivative in Modified Coinsurance Arrangement	87.0	96.3
Total Derivatives	87.0	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	217.9	173.6
Significant Unobservable Inputs (Level 3) \| Mortgage/Asset-backed Securities
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	19.7	0.7
Significant Unobservable Inputs (Level 3) \| All Other Corporate Bonds
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	528.3	829.7
Significant Unobservable Inputs (Level 3) \| Redeemable Preferred Stocks
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	22.6	21.7
United States Government and Government Agencies and Authorities
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	1,130.5	1,101.7
States, Municipalities, and Political Subdivisions
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	1,369.1	1,245.2
Foreign Governments
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	1,450.2	1,409.3
Public Utilities
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	9,886.4	9,684.2
Mortgage/Asset-backed Securities
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	3,239.2	3,385.5
All Other Corporate Bonds
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	23,635.5	23,154.7
Redeemable Preferred Stocks
Available for Sale Abstract [Abstract]
Fixed Maturity Securities	$ 57.5	$ 55.0

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