Fair Value Measurements by Input Level (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fixed Maturity Securities
Fixed Maturity Securities	$ 43,011.7	$ 45,457.8
Derivative Assets	30.9	19.5
Other Long-term Investments	702.9	646.8
Liabilities
Liabilities, Fair Value Disclosure	3,175.5	3,402.4
Fair Value, Inputs, Level 1
Liabilities
Liabilities, Fair Value Disclosure	1,429.8	1,171.8
Fair Value, Inputs, Level 2
Liabilities
Liabilities, Fair Value Disclosure	1,745.7	2,230.6
Fair Value, Inputs, Level 3
Liabilities
Liabilities, Fair Value Disclosure	0.0	0.0
Fair Value, Measurements, Recurring
Fixed Maturity Securities
Fixed Maturity Securities	43,011.7	45,457.8
Equity Securities	29.2	11.7
Other Long-term Investments	543.9	438.4
Assets, Fair Value Disclosure	43,555.6	45,896.2
Liabilities
Liabilities, Fair Value Disclosure	69.1	68.1
Fair Value, Measurements, Recurring | Interest Rate Swaps
Liabilities
Derivative Liabilities	5.2	5.1
Fair Value, Measurements, Recurring | Foreign Exchange Contracts
Liabilities
Derivative Liabilities	32.8	46.9
Fair Value, Measurements, Recurring | Credit Default Swaps
Liabilities
Derivative Liabilities		0.2
Fair Value, Measurements, Recurring | Embedded Derivative in Modified Coinsurance Arrangement
Liabilities
Derivative Liabilities	31.1	15.9
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	5,031.9	4,170.4
Equity Securities	0.0	0.2
Other Long-term Investments	0.0	0.2
Assets, Fair Value Disclosure	5,031.9	4,170.6
Liabilities
Liabilities, Fair Value Disclosure	0.0	0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Interest Rate Swaps
Liabilities
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Foreign Exchange Contracts
Liabilities
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Credit Default Swaps
Liabilities
Derivative Liabilities		0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Embedded Derivative in Modified Coinsurance Arrangement
Liabilities
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	36,346.8	39,906.8
Equity Securities	24.6	10.4
Other Long-term Investments	55.5	29.9
Assets, Fair Value Disclosure	36,402.3	39,936.7
Liabilities
Liabilities, Fair Value Disclosure	38.0	52.2
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Interest Rate Swaps
Liabilities
Derivative Liabilities	5.2	5.1
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Foreign Exchange Contracts
Liabilities
Derivative Liabilities	32.8	46.9
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Credit Default Swaps
Liabilities
Derivative Liabilities		0.2
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Embedded Derivative in Modified Coinsurance Arrangement
Liabilities
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	1,633.0	1,380.6
Equity Securities	4.6	1.1
Other Long-term Investments	4.6	1.1
Assets, Fair Value Disclosure	1,637.6	1,381.7
Liabilities
Liabilities, Fair Value Disclosure	31.1	15.9
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Interest Rate Swaps
Liabilities
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Foreign Exchange Contracts
Liabilities
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Credit Default Swaps
Liabilities
Derivative Liabilities		0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Embedded Derivative in Modified Coinsurance Arrangement
Liabilities
Derivative Liabilities	31.1	15.9
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Equity Securities	0.0	0.0
Other Long-term Investments	483.8	407.2
Assets, Fair Value Disclosure	483.8	407.2
Liabilities
Liabilities, Fair Value Disclosure	0.0	0.0
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share | Interest Rate Swaps
Liabilities
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share | Foreign Exchange Contracts
Liabilities
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share | Credit Default Swaps
Liabilities
Derivative Liabilities		0.0
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share | Embedded Derivative in Modified Coinsurance Arrangement
Liabilities
Derivative Liabilities	0.0	0.0
U.S. Government and Agencies
Fixed Maturity Securities
Fixed Maturity Securities	1,814.4	1,482.5
U.S. Government and Agencies | Fair Value, Measurements, Recurring
Fixed Maturity Securities
Fixed Maturity Securities	1,814.4	1,482.5
U.S. Government and Agencies | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	513.4	460.1
U.S. Government and Agencies | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	1,301.0	1,022.4
U.S. Government and Agencies | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
U.S. Government and Agencies | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
State and Municipal Securities
Fixed Maturity Securities
Fixed Maturity Securities	2,424.2	2,336.9
State and Municipal Securities | Fair Value, Measurements, Recurring
Fixed Maturity Securities
Fixed Maturity Securities	2,424.2	2,336.9
State and Municipal Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
State and Municipal Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	2,424.2	2,336.9
State and Municipal Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
State and Municipal Securities | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	983.7	863.9
Foreign Governments | Fair Value, Measurements, Recurring
Fixed Maturity Securities
Fixed Maturity Securities	983.7	863.9
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	952.3	863.9
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	31.4	0.0
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	7,412.8	8,236.5
Public Utilities | Fair Value, Measurements, Recurring
Fixed Maturity Securities
Fixed Maturity Securities	7,412.8	8,236.5
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	286.4	154.2
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	7,041.7	7,874.6
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	84.7	207.7
Public Utilities | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	1,582.7	1,973.6
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring
Fixed Maturity Securities
Fixed Maturity Securities	1,582.7	1,973.6
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	1,582.7	1,973.6
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Corporate | Fair Value, Measurements, Recurring
Fixed Maturity Securities
Fixed Maturity Securities	28,754.0	30,522.4
Corporate | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	4,232.1	3,556.1
Corporate | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	23,026.1	25,816.2
Corporate | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	1,495.8	1,150.1
Corporate | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	39.9	42.0
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring
Fixed Maturity Securities
Fixed Maturity Securities	39.9	42.0
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	18.8	19.2
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	21.1	22.8
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Foreign Exchange Contracts | Fair Value, Measurements, Recurring
Fixed Maturity Securities
Derivative Assets	30.4	19.5
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Derivative Assets	0.0	0.0
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Derivative Assets	30.4	19.5
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Derivative Assets	0.0	0.0
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fixed Maturity Securities
Derivative Assets	0.0	0.0
Credit Default Swaps | Fair Value, Measurements, Recurring
Fixed Maturity Securities
Derivative Assets	0.5
Credit Default Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Derivative Assets	0.0
Credit Default Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Derivative Assets	0.5
Credit Default Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Derivative Assets	0.0
Credit Default Swaps | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fixed Maturity Securities
Derivative Assets	0.0
Private Equity Funds of Funds
Fixed Maturity Securities
Alternative Investments	483.8
Private Equity Funds of Funds | Fair Value, Measurements, Recurring
Fixed Maturity Securities
Alternative Investments	483.8	407.2
Private Equity Funds of Funds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Alternative Investments	0.0	0.0
Private Equity Funds of Funds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Alternative Investments	0.0	0.0
Private Equity Funds of Funds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Alternative Investments	0.0	0.0
Private Equity Funds of Funds | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fixed Maturity Securities
Alternative Investments	483.8	407.2
Derivatives | Fair Value, Measurements, Recurring
Liabilities
Derivative Liabilities	69.1	68.1
Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Liabilities
Derivative Liabilities	0.0	0.0
Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Liabilities
Derivative Liabilities	38.0	52.2
Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Liabilities
Derivative Liabilities	31.1	15.9
Derivatives | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Liabilities
Derivative Liabilities	$ 0.0	$ 0.0