Fair Value Measurements by Input Level (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fixed Maturity Securities
Fixed Maturity Securities	$ 45,064.9	$ 42,344.4
Other Long-term Investments
Derivative Assets	28.0us-gaap_DerivativeAssets	10.8us-gaap_DerivativeAssets
Equity Securities	12.5	16.4
Other Liabilities
Derivative Liabilities	142.8us-gaap_DerivativeLiabilities	188.8us-gaap_DerivativeLiabilities
Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	2,960.3	2,523.3
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	40,130.0	37,480.6
Other Long-term Investments
Derivative Assets	28.0	10.8
Equity Securities	11.1	11.8
Other Liabilities
Derivative Liabilities	92.9	135.6
Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	1,974.6	2,340.5
Other Long-term Investments
Derivative Assets	0	0
Equity Securities	1.4	4.6
Other Liabilities
Derivative Liabilities	49.9	53.2
Interest Rate Swaps
Other Long-term Investments
Derivative Assets	5.7	9.2
Other Liabilities
Derivative Liabilities	20.8	35.0
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	5.7	9.2
Other Liabilities
Derivative Liabilities	20.8	35.0
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	22.3	1.6
Other Liabilities
Derivative Liabilities	70.9	98.7
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	22.3	1.6
Other Liabilities
Derivative Liabilities	70.9	98.7
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	1.2	1.9
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	1.2	1.9
Credit Default Swaps \| Fair Value, Inputs, Level 3
Other Liabilities
Derivative Liabilities	0	0
Embedded Derivative in Modified Coinsurance Arrangement
Other Liabilities
Derivative Liabilities	49.9	53.2
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	49.9	53.2
United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	1,238.5	1,196.1
United States Government and Government Agencies and Authorities \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	297.5	144.5
United States Government and Government Agencies and Authorities \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	941.0	1,051.6
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	2,121.5	1,783.2
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	1,981.4	1,608.1
States, Municipalities, and Political Subdivisions \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	140.1	175.1
Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	1,307.4	1,373.2
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	1,238.1	1,294.7
Foreign Governments \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	69.3	78.5
Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	8,550.6	7,997.2
Public Utilities \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	106.2	246.0
Public Utilities \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	8,129.4	7,611.9
Public Utilities \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	315.0	139.3
Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	2,431.8	2,039.3
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	2,431.8	2,038.8
Mortgage/Asset-backed Securities \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	0	0.5
Corporate Debt Securities
Fixed Maturity Securities
Fixed Maturity Securities	29,365.2	27,917.7
Corporate Debt Securities \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	2,556.6	2,132.8
Corporate Debt Securities \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	25,383.3	23,861.6
Corporate Debt Securities \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	1,425.3	1,923.3
Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	49.9	37.7
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	25.0	13.9
Redeemable Preferred Stocks \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	$ 24.9	$ 23.8

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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