Fair Value Measurements by Fair Value Heirarchy Input Level (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Fixed Maturity Securities
Fixed Maturity Securities	$ 45,422.6	$ 44,217.3
Other Long-term Investments
Derivative Assets	20.6	32.7
Equity Securities	1.3	1.2
Other Liabilities
Derivative Liabilities	77.4	99.5
Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	9,861.2	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	34,148.4	38,309.8
Other Long-term Investments
Equity Securities	0.0	0.0
Other Liabilities
Derivative Liabilities	52.2	52.8
Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	1,413.0	1,837.7
Other Long-term Investments
Equity Securities	1.3	1.2
Other Liabilities
Derivative Liabilities	25.2	46.7
Interest Rate Swaps
Other Liabilities
Derivative Liabilities	3.8	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	3.8	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	20.6	32.7
Other Liabilities
Derivative Liabilities	48.1	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	20.6	32.7
Other Liabilities
Derivative Liabilities	48.1	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	0.3	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	0.3	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	25.2	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	25.2	46.7
United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	1,501.2	1,382.4
United States Government and Government Agencies and Authorities \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	933.5	454.2
United States Government and Government Agencies and Authorities \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	567.7	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	2,278.6	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	2,190.0	2,068.5
States, Municipalities, and Political Subdivisions \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	88.6	89.5
Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	955.9	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	955.9	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	8,207.9	8,022.7
Public Utilities \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	580.3	108.5
Public Utilities \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	7,419.5	7,648.9
Public Utilities \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	208.1	265.3
Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	2,043.2	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	2,043.2	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	30,393.6	29,467.2
All Other Corporate Bonds \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	8,347.4	3,507.1
All Other Corporate Bonds \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	20,952.9	24,500.4
All Other Corporate Bonds \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	1,093.3	1,459.7
Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	42.2	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	19.2	18.7
Redeemable Preferred Stocks \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	$ 23.0	$ 23.2

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

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