Fair Value Measurements by Input Level (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Fixed Maturity Securities
Fixed Maturity Securities	$ 44,217.3	$ 43,354.4
Derivatives
Derivative Assets	32.7	49.8
Equity Securities	1.2	1.4
Derivatives
Derivative Liabilities	99.5	137.8
Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	4,069.8	2,926.8
Derivatives
Derivative Assets		0.0
Equity Securities	0.0	0.0
Derivatives
Derivative Liabilities	0.0	0.0
Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	38,309.8	38,546.4
Derivatives
Derivative Assets		49.8
Equity Securities	0.0	0.0
Derivatives
Derivative Liabilities	52.8	50.2
Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	1,837.7	1,881.2
Derivatives
Derivative Assets		0.0
Equity Securities	1.2	1.4
Derivatives
Derivative Liabilities	46.7	87.6
Interest Rate Swaps
Derivatives
Derivative Assets		2.4
Derivatives
Derivative Liabilities	7.6	12.3
Interest Rate Swaps \| Fair Value, Inputs, Level 1
Derivatives
Derivative Assets		0.0
Derivatives
Derivative Liabilities	0.0	0.0
Interest Rate Swaps \| Fair Value, Inputs, Level 2
Derivatives
Derivative Assets		2.4
Derivatives
Derivative Liabilities	7.6	12.3
Interest Rate Swaps \| Fair Value, Inputs, Level 3
Derivatives
Derivative Assets		0.0
Derivatives
Derivative Liabilities	0.0	0.0
Foreign Exchange Contracts
Derivatives
Derivative Assets	32.7	47.4
Derivatives
Derivative Liabilities	44.8	37.6
Foreign Exchange Contracts \| Fair Value, Inputs, Level 1
Derivatives
Derivative Assets	0.0	0.0
Derivatives
Derivative Liabilities	0.0	0.0
Foreign Exchange Contracts \| Fair Value, Inputs, Level 2
Derivatives
Derivative Assets	32.7	47.4
Derivatives
Derivative Liabilities	44.8	37.6
Foreign Exchange Contracts \| Fair Value, Inputs, Level 3
Derivatives
Derivative Assets	0.0	0.0
Derivatives
Derivative Liabilities	0.0	0.0
Credit Default Swaps
Derivatives
Derivative Liabilities	0.4	0.3
Credit Default Swaps \| Fair Value, Inputs, Level 1
Derivatives
Derivative Liabilities	0.0	0.0
Credit Default Swaps \| Fair Value, Inputs, Level 2
Derivatives
Derivative Liabilities	0.4	0.3
Credit Default Swaps \| Fair Value, Inputs, Level 3
Derivatives
Derivative Liabilities	0.0	0.0
Embedded Derivative in Modified Coinsurance Arrangement
Derivatives
Derivative Liabilities	46.7	87.6
Embedded Derivative in Modified Coinsurance Arrangement \| Fair Value, Inputs, Level 1
Derivatives
Derivative Liabilities	0.0	0.0
Embedded Derivative in Modified Coinsurance Arrangement \| Fair Value, Inputs, Level 2
Derivatives
Derivative Liabilities	0.0	0.0
Embedded Derivative in Modified Coinsurance Arrangement \| Fair Value, Inputs, Level 3
Derivatives
Derivative Liabilities	46.7	87.6
United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	1,382.4	1,465.2
United States Government and Government Agencies and Authorities \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	454.2	97.3
United States Government and Government Agencies and Authorities \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	928.2	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	2,158.0	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	2,068.5	1,994.9
States, Municipalities, and Political Subdivisions \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	89.5	122.2
Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	914.7	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	914.7	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	8,022.7	8,020.4
Public Utilities \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	108.5	59.1
Public Utilities \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	7,648.9	7,687.2
Public Utilities \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	265.3	274.1
Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	2,230.4	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	2,230.4	2,481.5
Mortgage/Asset-backed Securities \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Corporate Debt Securities
Fixed Maturity Securities
Fixed Maturity Securities	29,467.2	28,171.4
Corporate Debt Securities \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	3,507.1	2,770.4
Corporate Debt Securities \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	24,500.4	23,992.8
Corporate Debt Securities \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	1,459.7	1,408.2
Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	41.9	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	18.7	23.4
Redeemable Preferred Stocks \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	$ 23.2	$ 23.8

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

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