Fair Value Measurements by Fair Value Heirarchy Input Level (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Fixed Maturity Securities
Fixed Maturity Securities	$ 44,401.5	$ 45,064.9
Other Long-term Investments
Derivative Assets	57.1	28.0
Equity Securities	1.5	12.5
Other Liabilities
Derivative Liabilities	139.6	142.8
Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	7,077.3	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	35,769.9	40,130.0
Other Long-term Investments
Derivative Assets	57.1	28.0
Equity Securities	0.0	11.1
Other Liabilities
Derivative Liabilities	56.5	92.9
Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	1,554.3	1,974.6
Other Long-term Investments
Derivative Assets	0.0	0.0
Equity Securities	1.5	1.4
Other Liabilities
Derivative Liabilities	83.1	49.9
Interest Rate Swaps
Other Long-term Investments
Derivative Assets	4.9	5.7
Other Liabilities
Derivative Liabilities	12.9	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	4.9	5.7
Other Liabilities
Derivative Liabilities	12.9	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	52.2	22.3
Other Liabilities
Derivative Liabilities	42.5	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	52.2	22.3
Other Liabilities
Derivative Liabilities	42.5	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	1.1	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	1.1	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	83.1	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	83.1	49.9
United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	1,491.4	1,238.5
United States Government and Government Agencies and Authorities | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	678.6	297.5
United States Government and Government Agencies and Authorities | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	812.8	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	2,129.4	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	2,002.8	1,981.4
States, Municipalities, and Political Subdivisions | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	126.6	140.1
Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	1,224.6	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	1,156.3	1,238.1
Foreign Governments | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	68.3	69.3
Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	8,318.1	8,550.6
Public Utilities | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	405.0	106.2
Public Utilities | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	7,593.6	8,129.4
Public Utilities | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	319.5	315.0
Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	2,429.7	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	2,429.7	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	28,759.2	29,365.2
All Other Corporate Bonds | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	5,993.7	2,556.6
All Other Corporate Bonds | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	21,750.4	25,383.3
All Other Corporate Bonds | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	1,015.1	1,425.3
Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	49.1	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	24.3	25.0
Redeemable Preferred Stocks | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	$ 24.8	$ 24.9