Fair Value Measurements by Fair Value Heirarchy Input Level (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Fixed Maturity Securities
Fixed Maturity Securities	$ 45,118.6	$ 44,217.3
Other Long-term Investments
Derivative Assets	27.0	32.7
Equity Securities	1.2	1.2
Other Liabilities
Derivative Liabilities	84.5	99.5
Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	8,873.8	4,069.8
Other Long-term Investments
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	35,139.2	38,309.8
Other Long-term Investments
Equity Securities	0.0	0.0
Other Liabilities
Derivative Liabilities	52.6	52.8
Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	1,105.6	1,837.7
Other Long-term Investments
Equity Securities	1.2	1.2
Other Liabilities
Derivative Liabilities	31.9	46.7
Interest Rate Swaps
Other Liabilities
Derivative Liabilities	4.3	7.6
Interest Rate Swaps \| Fair Value, Inputs, Level 1
Other Liabilities
Derivative Liabilities	0.0	0.0
Interest Rate Swaps \| Fair Value, Inputs, Level 2
Other Liabilities
Derivative Liabilities	4.3	7.6
Interest Rate Swaps \| Fair Value, Inputs, Level 3
Other Liabilities
Derivative Liabilities	0.0	0.0
Foreign Exchange Contracts
Other Long-term Investments
Derivative Assets	27.0	32.7
Other Liabilities
Derivative Liabilities	47.8	44.8
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	27.0	32.7
Other Liabilities
Derivative Liabilities	47.8	44.8
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	0.5	0.4
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	0.5	0.4
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	31.9	46.7
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	31.9	46.7
United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	1,436.0	1,382.4
United States Government and Government Agencies and Authorities \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	179.4	454.2
United States Government and Government Agencies and Authorities \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	1,256.6	928.2
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	2,252.4	2,158.0
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	2,215.6	2,068.5
States, Municipalities, and Political Subdivisions \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	36.8	89.5
Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	934.0	914.7
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	934.0	914.7
Foreign Governments \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	8,212.4	8,022.7
Public Utilities \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	706.9	108.5
Public Utilities \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	7,313.0	7,648.9
Public Utilities \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	192.5	265.3
Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	2,102.5	2,230.4
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	2,102.5	2,230.4
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	30,139.2	29,467.2
All Other Corporate Bonds \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	7,987.5	3,507.1
All Other Corporate Bonds \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	21,298.5	24,500.4
All Other Corporate Bonds \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	853.2	1,459.7
Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	42.1	41.9
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	19.0	18.7
Redeemable Preferred Stocks \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	$ 23.1	$ 23.2

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

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