Fair Value Measurements by Input Level (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fixed Maturity Securities
Fixed Maturity Securities	$ 45,457.8	$ 44,217.3
Other Long-term Investments
Equity Securities	11.7	1.2
Derivatives
Derivative Assets	19.5	32.7
Derivatives
Derivative Liabilities	68.1	99.5
Interest Rate Swaps
Derivatives
Derivative Liabilities	5.1	7.6
Foreign Exchange Contracts
Derivatives
Derivative Assets	19.5	32.7
Derivatives
Derivative Liabilities	46.9	44.8
Credit Default Swaps
Derivatives
Derivative Liabilities	0.2	0.4
Embedded Derivative in Modified Coinsurance Arrangement
Derivatives
Derivative Liabilities	15.9	46.7
United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	1,482.5	1,382.4
States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Fixed Maturity Securities	2,336.9	2,158.0
Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	863.9	914.7
Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	8,236.5	8,022.7
Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	1,973.6	2,230.4
Corporate Debt Securities
Fixed Maturity Securities
Fixed Maturity Securities	30,522.4	29,467.2
Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	42.0	41.9
Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	4,170.4	4,069.8
Other Long-term Investments
Equity Securities	0.2	0.0
Derivatives
Derivative Liabilities	0.0	0.0
Fair Value, Inputs, Level 1 | Interest Rate Swaps
Derivatives
Derivative Liabilities	0.0	0.0
Fair Value, Inputs, Level 1 | Foreign Exchange Contracts
Derivatives
Derivative Assets	0.0	0.0
Derivatives
Derivative Liabilities	0.0	0.0
Fair Value, Inputs, Level 1 | Credit Default Swaps
Derivatives
Derivative Liabilities	0.0	0.0
Fair Value, Inputs, Level 1 | Embedded Derivative in Modified Coinsurance Arrangement
Derivatives
Derivative Liabilities	0.0	0.0
Fair Value, Inputs, Level 1 | United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	460.1	454.2
Fair Value, Inputs, Level 1 | States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Fair Value, Inputs, Level 1 | Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Fair Value, Inputs, Level 1 | Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	154.2	108.5
Fair Value, Inputs, Level 1 | Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Fair Value, Inputs, Level 1 | Corporate Debt Securities
Fixed Maturity Securities
Fixed Maturity Securities	3,556.1	3,507.1
Fair Value, Inputs, Level 1 | Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	39,906.8	38,309.8
Other Long-term Investments
Equity Securities	10.4	0.0
Derivatives
Derivative Liabilities	52.2	52.8
Fair Value, Inputs, Level 2 | Interest Rate Swaps
Derivatives
Derivative Liabilities	5.1	7.6
Fair Value, Inputs, Level 2 | Foreign Exchange Contracts
Derivatives
Derivative Assets	19.5	32.7
Derivatives
Derivative Liabilities	46.9	44.8
Fair Value, Inputs, Level 2 | Credit Default Swaps
Derivatives
Derivative Liabilities	0.2	0.4
Fair Value, Inputs, Level 2 | Embedded Derivative in Modified Coinsurance Arrangement
Derivatives
Derivative Liabilities	0.0	0.0
Fair Value, Inputs, Level 2 | United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	1,022.4	928.2
Fair Value, Inputs, Level 2 | States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Fixed Maturity Securities	2,336.9	2,068.5
Fair Value, Inputs, Level 2 | Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	863.9	914.7
Fair Value, Inputs, Level 2 | Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	7,874.6	7,648.9
Fair Value, Inputs, Level 2 | Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	1,973.6	2,230.4
Fair Value, Inputs, Level 2 | Corporate Debt Securities
Fixed Maturity Securities
Fixed Maturity Securities	25,816.2	24,500.4
Fair Value, Inputs, Level 2 | Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	19.2	18.7
Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	1,380.6	1,837.7
Other Long-term Investments
Equity Securities	1.1	1.2
Derivatives
Derivative Liabilities	15.9	46.7
Fair Value, Inputs, Level 3 | Interest Rate Swaps
Derivatives
Derivative Liabilities	0.0	0.0
Fair Value, Inputs, Level 3 | Foreign Exchange Contracts
Derivatives
Derivative Assets	0.0	0.0
Derivatives
Derivative Liabilities	0.0	0.0
Fair Value, Inputs, Level 3 | Credit Default Swaps
Derivatives
Derivative Liabilities	0.0	0.0
Fair Value, Inputs, Level 3 | Embedded Derivative in Modified Coinsurance Arrangement
Derivatives
Derivative Liabilities	15.9	46.7
Fair Value, Inputs, Level 3 | United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Fair Value, Inputs, Level 3 | States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Fixed Maturity Securities	0.0	89.5
Fair Value, Inputs, Level 3 | Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Fair Value, Inputs, Level 3 | Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	207.7	265.3
Fair Value, Inputs, Level 3 | Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Fair Value, Inputs, Level 3 | Corporate Debt Securities
Fixed Maturity Securities
Fixed Maturity Securities	1,150.1	1,459.7
Fair Value, Inputs, Level 3 | Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	$ 22.8	$ 23.2