Fair Value Measurements by Input Level (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fixed Maturity Securities
Fixed Maturity Securities	$ 42,594.2	$ 44,973.0
Other Long-term Investments
Derivative Assets	36.4	81.6
Equity Securities
Equity Securities	14.3	14.6
Other Liabilities
Derivative Liabilities	202.7	254.4
Interest Rate Swaps
Other Long-term Investments
Derivative Assets	35.7	76.5
Other Liabilities
Derivative Liabilities	37.5	31.7
Foreign Exchange Contracts
Other Long-term Investments
Derivative Assets	0.7	5.1
Other Liabilities
Derivative Liabilities	99.5	138.8
Credit Default Swaps
Other Liabilities
Derivative Liabilities	0.6
Embedded Derivative in Modified Coinsurance Arrangement
Other Liabilities
Derivative Liabilities	65.1	83.9
United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	1,236.9	1,348.8
States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Fixed Maturity Securities	1,723.0	1,806.8
Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	1,355.4	1,507.0
Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	10,568.2	11,144.2
Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	1,857.0	2,216.5
All Other Corporate Bonds
Fixed Maturity Securities
Fixed Maturity Securities	25,815.6	26,910.4
Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	38.1	39.3
Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	6,238.1	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 | Interest Rate Swaps
Other Long-term Investments
Derivative Assets	0	0
Other Liabilities
Derivative Liabilities	0	0
Fair Value, Inputs, Level 1 | Foreign Exchange Contracts
Other Long-term Investments
Derivative Assets	0	0
Other Liabilities
Derivative Liabilities	0	0
Fair Value, Inputs, Level 1 | Credit Default Swaps
Other Liabilities
Derivative Liabilities	0
Fair Value, Inputs, Level 1 | Embedded Derivative in Modified Coinsurance Arrangement
Other Liabilities
Derivative Liabilities	0	0
Fair Value, Inputs, Level 1 | United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	280.0	104.1
Fair Value, Inputs, Level 1 | States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Fixed Maturity Securities	31.8	53.0
Fair Value, Inputs, Level 1 | Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	0	0
Fair Value, Inputs, Level 1 | Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	894.2	84.2
Fair Value, Inputs, Level 1 | Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	0	0
Fair Value, Inputs, Level 1 | All Other Corporate Bonds
Fixed Maturity Securities
Fixed Maturity Securities	5,032.1	1,977.1
Fair Value, Inputs, Level 1 | Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	0	0
Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	34,688.3	40,766.3
Other Long-term Investments
Derivative Assets	36.4	81.6
Equity Securities
Equity Securities	9.9	10.3
Other Liabilities
Derivative Liabilities	137.6	170.5
Fair Value, Inputs, Level 2 | Interest Rate Swaps
Other Long-term Investments
Derivative Assets	35.7	76.5
Other Liabilities
Derivative Liabilities	37.5	31.7
Fair Value, Inputs, Level 2 | Foreign Exchange Contracts
Other Long-term Investments
Derivative Assets	0.7	5.1
Other Liabilities
Derivative Liabilities	99.5	138.8
Fair Value, Inputs, Level 2 | Credit Default Swaps
Other Liabilities
Derivative Liabilities	0.6
Fair Value, Inputs, Level 2 | Embedded Derivative in Modified Coinsurance Arrangement
Other Liabilities
Derivative Liabilities	0	0
Fair Value, Inputs, Level 2 | United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	956.9	1,244.7
Fair Value, Inputs, Level 2 | States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Fixed Maturity Securities	1,539.2	1,625.1
Fair Value, Inputs, Level 2 | Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	1,275.4	1,424.9
Fair Value, Inputs, Level 2 | Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	9,319.8	10,485.6
Fair Value, Inputs, Level 2 | Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	1,842.1	2,216.0
Fair Value, Inputs, Level 2 | All Other Corporate Bonds
Fixed Maturity Securities
Fixed Maturity Securities	19,741.0	23,755.5
Fair Value, Inputs, Level 2 | Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	13.9	14.5
Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	1,667.8	1,988.3
Other Long-term Investments
Derivative Assets	0	0
Equity Securities
Equity Securities	4.4	4.3
Other Liabilities
Derivative Liabilities	65.1	83.9
Fair Value, Inputs, Level 3 | Interest Rate Swaps
Other Long-term Investments
Derivative Assets	0	0
Other Liabilities
Derivative Liabilities	0	0
Fair Value, Inputs, Level 3 | Foreign Exchange Contracts
Other Long-term Investments
Derivative Assets	0	0
Other Liabilities
Derivative Liabilities	0	0
Fair Value, Inputs, Level 3 | Credit Default Swaps
Other Liabilities
Derivative Liabilities	0
Fair Value, Inputs, Level 3 | Embedded Derivative in Modified Coinsurance Arrangement
Other Liabilities
Derivative Liabilities	65.1	83.9
Fair Value, Inputs, Level 3 | United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	0	0
Fair Value, Inputs, Level 3 | States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Fixed Maturity Securities	152.0	128.7
Fair Value, Inputs, Level 3 | Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	80.0	82.1
Fair Value, Inputs, Level 3 | Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	354.2	574.4
Fair Value, Inputs, Level 3 | Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	14.9	0.5
Fair Value, Inputs, Level 3 | All Other Corporate Bonds
Fixed Maturity Securities
Fixed Maturity Securities	1,042.5	1,177.8
Fair Value, Inputs, Level 3 | Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	$ 24.2	$ 24.8