Fair Value Measurements by Fair Value Heirarchy Input Level (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Fixed Maturity Securities
Fixed Maturity Securities	$ 46,315.5	$ 43,354.4
Other Long-term Investments
Derivative Assets	37.1	49.8
Equity Securities	1.2	1.4
Other Liabilities
Derivative Liabilities	129.0	137.8
Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	9,152.9	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	35,669.8	38,546.4
Other Long-term Investments
Derivative Assets	37.1	49.8
Equity Securities	0.0	0.0
Other Liabilities
Derivative Liabilities	55.7	50.2
Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	1,492.8	1,881.2
Other Long-term Investments
Derivative Assets	0.0	0.0
Equity Securities	1.2	1.4
Other Liabilities
Derivative Liabilities	73.3	87.6
Interest Rate Swaps
Other Long-term Investments
Derivative Assets	3.2	2.4
Other Liabilities
Derivative Liabilities	6.1	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	3.2	2.4
Other Liabilities
Derivative Liabilities	6.1	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	33.9	47.4
Other Liabilities
Derivative Liabilities	49.2	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	33.9	47.4
Other Liabilities
Derivative Liabilities	49.2	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	0.4	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	0.4	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	73.3	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	73.3	87.6
United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	1,452.2	1,465.2
United States Government and Government Agencies and Authorities \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	285.5	97.3
United States Government and Government Agencies and Authorities \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	1,166.7	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	2,296.7	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	2,204.6	1,994.9
States, Municipalities, and Political Subdivisions \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	92.1	122.2
Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	981.7	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	981.7	998.7
Foreign Governments \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	0.0	52.9
Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	8,422.6	8,020.4
Public Utilities \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	632.5	59.1
Public Utilities \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	7,385.8	7,687.2
Public Utilities \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	404.3	274.1
Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	2,386.2	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	2,386.2	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	30,729.1	28,171.4
All Other Corporate Bonds \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	8,234.9	2,770.4
All Other Corporate Bonds \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	21,521.3	23,992.8
All Other Corporate Bonds \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	972.9	1,408.2
Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	47.0	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	23.5	23.4
Redeemable Preferred Stocks \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	$ 23.5	$ 23.8

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

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