Fair Value Measurements by Fair Value Heirarchy Input Level (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Fixed Maturity Securities
Fixed Maturity Securities	$ 44,699.2	$ 43,354.4
Other Long-term Investments
Derivative Assets	40.1	49.8
Equity Securities	1.5	1.4
Other Liabilities
Derivative Liabilities	151.2	137.8
Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	9,332.5	2,926.8
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	33,855.8	38,546.4
Other Long-term Investments
Derivative Assets	40.1	49.8
Equity Securities	0.3	0.0
Other Liabilities
Derivative Liabilities	58.0	50.2
Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	1,510.9	1,881.2
Other Long-term Investments
Derivative Assets	0.0	0.0
Equity Securities	1.2	1.4
Other Liabilities
Derivative Liabilities	93.2	87.6
Interest Rate Swaps
Other Long-term Investments
Derivative Assets	5.3	2.4
Other Liabilities
Derivative Liabilities	7.8	12.3
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	5.3	2.4
Other Liabilities
Derivative Liabilities	7.8	12.3
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	34.8	47.4
Other Liabilities
Derivative Liabilities	50.1	37.6
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	34.8	47.4
Other Liabilities
Derivative Liabilities	50.1	37.6
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	0.1	0.3
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	0.1	0.3
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	93.2	87.6
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	93.2	87.6
United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	1,586.3	1,465.2
United States Government and Government Agencies and Authorities \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	689.4	97.3
United States Government and Government Agencies and Authorities \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	896.9	1,367.9
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	2,176.6	2,117.1
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	2,087.9	1,994.9
States, Municipalities, and Political Subdivisions \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	88.7	122.2
Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	1,077.9	1,051.6
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	1,024.0	998.7
Foreign Governments \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	53.9	52.9
Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	8,243.2	8,020.4
Public Utilities \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	569.1	59.1
Public Utilities \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	7,403.9	7,687.2
Public Utilities \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	270.2	274.1
Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	2,480.4	2,481.5
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	2,480.4	2,481.5
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	29,087.5	28,171.4
All Other Corporate Bonds \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	8,074.0	2,770.4
All Other Corporate Bonds \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	19,938.8	23,992.8
All Other Corporate Bonds \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	1,074.7	1,408.2
Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	47.3	47.2
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	23.9	23.4
Redeemable Preferred Stocks \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	$ 23.4	$ 23.8

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

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