Fair Value Measurements by Input Level (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Fixed Maturity Securities
Fixed Maturity Securities	$ 42,597.6	$ 44,973.0
Other Long-term Investments
Derivative Assets	24.5	81.6
Equity Securities
Equity Securities	15.5	14.6
Other Liabilities
Derivative Liabilities	208.3	254.4
Interest Rate Swaps
Other Long-term Investments
Derivative Assets	22.5	76.5
Other Liabilities
Derivative Liabilities	33.8	31.7
Foreign Exchange Contracts
Other Long-term Investments
Derivative Assets	2.0	5.1
Other Liabilities
Derivative Liabilities	107.6	138.8
Credit Default Swaps
Other Liabilities
Derivative Liabilities	1.5
Embedded Derivative in Modified Coinsurance Arrangement
Other Liabilities
Derivative Liabilities	65.4	83.9
United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	1,223.2	1,348.8
States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Fixed Maturity Securities	1,769.8	1,806.8
Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	1,375.3	1,507.0
Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	10,556.7	11,144.2
Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	2,161.6	2,216.5
All Other Corporate Bonds
Fixed Maturity Securities
Fixed Maturity Securities	25,472.8	26,910.4
Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	38.2	39.3
Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	4,368.5	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	135.1	104.1
Fair Value, Inputs, Level 1 | States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Fixed Maturity Securities	0	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	692.0	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	3,541.4	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	36,469.7	40,766.3
Other Long-term Investments
Derivative Assets	24.5	81.6
Equity Securities
Equity Securities	11.1	10.3
Other Liabilities
Derivative Liabilities	142.9	170.5
Fair Value, Inputs, Level 2 | Interest Rate Swaps
Other Long-term Investments
Derivative Assets	22.5	76.5
Other Liabilities
Derivative Liabilities	33.8	31.7
Fair Value, Inputs, Level 2 | Foreign Exchange Contracts
Other Long-term Investments
Derivative Assets	2.0	5.1
Other Liabilities
Derivative Liabilities	107.6	138.8
Fair Value, Inputs, Level 2 | Credit Default Swaps
Other Liabilities
Derivative Liabilities	1.5
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	1,088.1	1,244.7
Fair Value, Inputs, Level 2 | States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Fixed Maturity Securities	1,617.9	1,625.1
Fair Value, Inputs, Level 2 | Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	1,296.7	1,424.9
Fair Value, Inputs, Level 2 | Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	9,572.9	10,485.6
Fair Value, Inputs, Level 2 | Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	2,161.1	2,216.0
Fair Value, Inputs, Level 2 | All Other Corporate Bonds
Fixed Maturity Securities
Fixed Maturity Securities	20,718.9	23,755.5
Fair Value, Inputs, Level 2 | Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	14.1	14.5
Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	1,759.4	1,988.3
Other Long-term Investments
Derivative Assets	0	0
Equity Securities
Equity Securities	4.4	4.3
Other Liabilities
Derivative Liabilities	65.4	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	65.4	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	151.9	128.7
Fair Value, Inputs, Level 3 | Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	78.6	82.1
Fair Value, Inputs, Level 3 | Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	291.8	574.4
Fair Value, Inputs, Level 3 | Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	0.5	0.5
Fair Value, Inputs, Level 3 | All Other Corporate Bonds
Fixed Maturity Securities
Fixed Maturity Securities	1,212.5	1,177.8
Fair Value, Inputs, Level 3 | Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	$ 24.1	$ 24.8