Fair Value Measurements by Fair Value Heirarchy Input Level (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Fixed Maturity Securities
Fixed Maturity Securities	$ 44,570.4	$ 45,064.9
Other Long-term Investments
Derivative Assets	39.3	28.0
Equity Securities	1.4	12.5
Other Liabilities
Derivative Liabilities	130.5	142.8
Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	7,665.6	2,960.3
Other Long-term Investments
Derivative Assets	0.0	0.0
Equity Securities	0.0	0.0
Other Liabilities
Derivative Liabilities	0.0	0.0
Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	34,993.8	40,130.0
Other Long-term Investments
Derivative Assets	39.3	28.0
Equity Securities	0.0	11.1
Other Liabilities
Derivative Liabilities	74.7	92.9
Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	1,911.0	1,974.6
Other Long-term Investments
Derivative Assets	0.0	0.0
Equity Securities	1.4	1.4
Other Liabilities
Derivative Liabilities	55.8	49.9
Interest Rate Swaps
Other Long-term Investments
Derivative Assets	4.8	5.7
Other Liabilities
Derivative Liabilities	17.4	20.8
Interest Rate Swaps | Fair Value, Inputs, Level 1
Other Long-term Investments
Derivative Assets	0.0	0.0
Other Liabilities
Derivative Liabilities	0.0	0.0
Interest Rate Swaps | Fair Value, Inputs, Level 2
Other Long-term Investments
Derivative Assets	4.8	5.7
Other Liabilities
Derivative Liabilities	17.4	20.8
Interest Rate Swaps | Fair Value, Inputs, Level 3
Other Long-term Investments
Derivative Assets	0.0	0.0
Other Liabilities
Derivative Liabilities	0.0	0.0
Foreign Exchange Contracts
Other Long-term Investments
Derivative Assets	34.5	22.3
Other Liabilities
Derivative Liabilities	56.0	70.9
Foreign Exchange Contracts | Fair Value, Inputs, Level 1
Other Long-term Investments
Derivative Assets	0.0	0.0
Other Liabilities
Derivative Liabilities	0.0	0.0
Foreign Exchange Contracts | Fair Value, Inputs, Level 2
Other Long-term Investments
Derivative Assets	34.5	22.3
Other Liabilities
Derivative Liabilities	56.0	70.9
Foreign Exchange Contracts | Fair Value, Inputs, Level 3
Other Long-term Investments
Derivative Assets	0.0	0.0
Other Liabilities
Derivative Liabilities	0.0	0.0
Credit Default Swaps
Other Liabilities
Derivative Liabilities	1.3	1.2
Credit Default Swaps | Fair Value, Inputs, Level 1
Other Liabilities
Derivative Liabilities	0.0	0.0
Credit Default Swaps | Fair Value, Inputs, Level 2
Other Liabilities
Derivative Liabilities	1.3	1.2
Credit Default Swaps | Fair Value, Inputs, Level 3
Other Liabilities
Derivative Liabilities	0.0	0.0
Embedded Derivative in Modified Coinsurance Arrangement
Other Liabilities
Derivative Liabilities	55.8	49.9
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Inputs, Level 1
Other Liabilities
Derivative Liabilities	0.0	0.0
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Inputs, Level 2
Other Liabilities
Derivative Liabilities	0.0	0.0
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Inputs, Level 3
Other Liabilities
Derivative Liabilities	55.8	49.9
United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	1,453.8	1,238.5
United States Government and Government Agencies and Authorities | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	664.4	297.5
United States Government and Government Agencies and Authorities | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	789.4	941.0
United States Government and Government Agencies and Authorities | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Fixed Maturity Securities	2,073.7	2,121.5
States, Municipalities, and Political Subdivisions | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
States, Municipalities, and Political Subdivisions | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	1,937.0	1,981.4
States, Municipalities, and Political Subdivisions | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	136.7	140.1
Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	1,271.5	1,307.4
Foreign Governments | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Foreign Governments | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	1,201.7	1,238.1
Foreign Governments | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	69.8	69.3
Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	8,246.8	8,550.6
Public Utilities | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	455.4	106.2
Public Utilities | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	7,433.6	8,129.4
Public Utilities | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	357.8	315.0
Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	2,579.5	2,431.8
Mortgage/Asset-backed Securities | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Mortgage/Asset-backed Securities | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	2,579.5	2,431.8
Mortgage/Asset-backed Securities | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
All Other Corporate Bonds
Fixed Maturity Securities
Fixed Maturity Securities	28,895.9	29,365.2
All Other Corporate Bonds | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	6,545.8	2,556.6
All Other Corporate Bonds | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	21,028.2	25,383.3
All Other Corporate Bonds | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	1,321.9	1,425.3
Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	49.2	49.9
Redeemable Preferred Stocks | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Redeemable Preferred Stocks | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	24.4	25.0
Redeemable Preferred Stocks | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	$ 24.8	$ 24.9