Fair Value Measurements by Input Level (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Fixed Maturity Securities
Fixed Maturity Securities	$ 43,354.4	$ 45,064.9
Other Long-term Investments
Derivative Assets	49.8	28.0
Equity Securities	1.4	12.5
Other Liabilities
Derivative Liabilities	137.8	142.8
Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	2,926.8	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	38,546.4	40,130.0
Other Long-term Investments
Derivative Assets	49.8	28.0
Equity Securities	0.0	11.1
Other Liabilities
Derivative Liabilities	50.2	92.9
Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	1,881.2	1,974.6
Other Long-term Investments
Derivative Assets	0.0	0.0
Equity Securities	1.4	1.4
Other Liabilities
Derivative Liabilities	87.6	49.9
Interest Rate Swaps
Other Long-term Investments
Derivative Assets	2.4	5.7
Other Liabilities
Derivative Liabilities	12.3	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	2.4	5.7
Other Liabilities
Derivative Liabilities	12.3	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	47.4	22.3
Other Liabilities
Derivative Liabilities	37.6	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	47.4	22.3
Other Liabilities
Derivative Liabilities	37.6	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	0.3	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	0.3	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	87.6	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	87.6	49.9
United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	1,465.2	1,238.5
United States Government and Government Agencies and Authorities \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	97.3	297.5
United States Government and Government Agencies and Authorities \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	1,367.9	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	2,117.1	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	1,994.9	1,981.4
States, Municipalities, and Political Subdivisions \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	122.2	140.1
Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	1,051.6	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	998.7	1,238.1
Foreign Governments \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	52.9	69.3
Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	8,020.4	8,550.6
Public Utilities \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	59.1	106.2
Public Utilities \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	7,687.2	8,129.4
Public Utilities \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	274.1	315.0
Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	2,481.5	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	2,481.5	2,431.8
Mortgage/Asset-backed Securities \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Corporate Debt Securities
Fixed Maturity Securities
Fixed Maturity Securities	28,171.4	29,365.2
Corporate Debt Securities \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	2,770.4	2,556.6
Corporate Debt Securities \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	23,992.8	25,383.3
Corporate Debt Securities \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	1,408.2	1,425.3
Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	47.2	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	23.4	25.0
Redeemable Preferred Stocks \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	$ 23.8	$ 24.9

Amount of debt securities categorized neither as held-to-maturity nor trading.

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