Fair Value Measurements by Fair Value Heirarchy Input Level (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Fixed Maturity Securities
Fixed Maturity Securities	$ 45,852.7	$ 45,064.9
Other Long-term Investments
Derivative Assets	53.2us-gaap_DerivativeAssets	28.0us-gaap_DerivativeAssets
Equity Securities	11.9	12.5
Other Liabilities
Derivative Liabilities	125.6us-gaap_DerivativeLiabilities	142.8us-gaap_DerivativeLiabilities
Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	7,865.9	2,960.3
Other Long-term Investments
Derivative Assets	0	0
Equity Securities	0	0
Other Liabilities
Derivative Liabilities	0	0
Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	35,563.0	40,130.0
Other Long-term Investments
Derivative Assets	53.2	28.0
Equity Securities	10.5	11.1
Other Liabilities
Derivative Liabilities	71.8	92.9
Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	2,423.8	1,974.6
Other Long-term Investments
Derivative Assets	0	0
Equity Securities	1.4	1.4
Other Liabilities
Derivative Liabilities	53.8	49.9
Interest Rate Swaps
Other Long-term Investments
Derivative Assets	5.6	5.7
Other Liabilities
Derivative Liabilities	16.4	20.8
Interest Rate Swaps \| Fair Value, Inputs, Level 1
Other Long-term Investments
Derivative Assets	0	0
Other Liabilities
Derivative Liabilities	0	0
Interest Rate Swaps \| Fair Value, Inputs, Level 2
Other Long-term Investments
Derivative Assets	5.6	5.7
Other Liabilities
Derivative Liabilities	16.4	20.8
Interest Rate Swaps \| Fair Value, Inputs, Level 3
Other Long-term Investments
Derivative Assets	0	0
Other Liabilities
Derivative Liabilities	0	0
Foreign Exchange Contracts
Other Long-term Investments
Derivative Assets	47.6	22.3
Other Liabilities
Derivative Liabilities	54.1	70.9
Foreign Exchange Contracts \| Fair Value, Inputs, Level 1
Other Long-term Investments
Derivative Assets	0	0
Other Liabilities
Derivative Liabilities	0	0
Foreign Exchange Contracts \| Fair Value, Inputs, Level 2
Other Long-term Investments
Derivative Assets	47.6	22.3
Other Liabilities
Derivative Liabilities	54.1	70.9
Foreign Exchange Contracts \| Fair Value, Inputs, Level 3
Other Long-term Investments
Derivative Assets	0	0
Other Liabilities
Derivative Liabilities	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
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
Embedded Derivative in Modified Coinsurance Arrangement
Other Liabilities
Derivative Liabilities	53.8	49.9
Embedded Derivative in Modified Coinsurance Arrangement \| Fair Value, Inputs, Level 1
Other Liabilities
Derivative Liabilities	0	0
Embedded Derivative in Modified Coinsurance Arrangement \| Fair Value, Inputs, Level 2
Other Liabilities
Derivative Liabilities	0	0
Embedded Derivative in Modified Coinsurance Arrangement \| Fair Value, Inputs, Level 3
Other Liabilities
Derivative Liabilities	53.8	49.9
United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	1,413.4	1,238.5
United States Government and Government Agencies and Authorities \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	673.8	297.5
United States Government and Government Agencies and Authorities \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	739.6	941.0
United States Government and Government Agencies and Authorities \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	0	0
States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Fixed Maturity Securities	2,162.1	2,121.5
States, Municipalities, and Political Subdivisions \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	0	0
States, Municipalities, and Political Subdivisions \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	2,017.9	1,981.4
States, Municipalities, and Political Subdivisions \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	144.2	140.1
Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	1,243.9	1,307.4
Foreign Governments \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	0	0
Foreign Governments \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	1,174.8	1,238.1
Foreign Governments \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	69.1	69.3
Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	8,666.1	8,550.6
Public Utilities \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	690.9	106.2
Public Utilities \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	7,425.4	8,129.4
Public Utilities \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	549.8	315.0
Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	2,415.3	2,431.8
Mortgage/Asset-backed Securities \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	0	0
Mortgage/Asset-backed Securities \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	2,415.3	2,431.8
Mortgage/Asset-backed Securities \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	0	0
Corporate Debt Securities
Fixed Maturity Securities
Fixed Maturity Securities	29,902.1	29,365.2
Corporate Debt Securities \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	6,501.2	2,556.6
Corporate Debt Securities \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	21,765.1	25,383.3
Corporate Debt Securities \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	1,635.8	1,425.3
Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	49.8	49.9
Redeemable Preferred Stocks \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	0	0
Redeemable Preferred Stocks \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	24.9	25.0
Redeemable Preferred Stocks \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	$ 24.9	$ 24.9

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

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