Fair Value Measurements by Input Level (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fixed Maturity Securities
Fixed Maturity Securities	$ 47,443.7	$ 43,011.7
Derivative Assets	27.5	30.9
Equity Securities	32.6	29.2
Other Long-term Investments	844.2	702.9
Assets, Fair Value Disclosure	48,120.5	43,555.6
Liabilities
Derivative Liabilities	57.4	69.1
Liabilities, Fair Value Disclosure	57.4	69.1
Interest Rate Swaps
Liabilities
Derivative Liabilities	0.6	5.2
Foreign Exchange Contracts
Liabilities
Derivative Liabilities	34.0	32.8
Embedded Derivative in Modified Coinsurance Arrangement
Liabilities
Derivative Liabilities	22.8	31.1
Fair Value, Measurements, Recurring
Fixed Maturity Securities
Other Long-term Investments	676.8	543.9
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	4,698.3	5,031.9
Derivative Assets	0.0	0.0
Equity Securities	0.0	0.0
Other Long-term Investments	0.0	0.0
Assets, Fair Value Disclosure	4,698.3	5,031.9
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 | 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	42,032.6	36,346.8
Derivative Assets	27.5	30.9
Equity Securities	28.0	24.6
Other Long-term Investments	55.5	55.5
Assets, Fair Value Disclosure	42,088.1	36,402.3
Liabilities
Liabilities, Fair Value Disclosure	34.6	38.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Interest Rate Swaps
Liabilities
Derivative Liabilities	0.6	5.2
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Foreign Exchange Contracts
Liabilities
Derivative Liabilities	34.0	32.8
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	712.8	1,633.0
Derivative Assets	0.0	0.0
Equity Securities	4.6	4.6
Other Long-term Investments	4.6	4.6
Assets, Fair Value Disclosure	717.4	1,637.6
Liabilities
Liabilities, Fair Value Disclosure	22.8	31.1
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 | Embedded Derivative in Modified Coinsurance Arrangement
Liabilities
Derivative Liabilities	22.8	31.1
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Derivative Assets	0.0	0.0
Equity Securities	0.0	0.0
Other Long-term Investments	616.7	483.8
Assets, Fair Value Disclosure	616.7	483.8
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 | Embedded Derivative in Modified Coinsurance Arrangement
Liabilities
Derivative Liabilities	0.0	0.0
U.S. Government and Agencies
Fixed Maturity Securities
Fixed Maturity Securities	1,401.7	1,814.4
U.S. Government and Agencies | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	412.8	513.4
U.S. Government and Agencies | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	988.9	1,301.0
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	3,363.4	2,424.2
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	3,321.6	2,424.2
State and Municipal Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	41.8	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	1,017.7	983.7
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	995.9	952.3
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	21.8	31.4
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	7,732.2	7,412.8
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	171.1	286.4
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	7,546.5	7,041.7
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	14.6	84.7
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	1,478.7	1,582.7
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	1,444.6	1,582.7
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	34.1	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
Fixed Maturity Securities
Fixed Maturity Securities	32,410.4	28,754.0
Corporate | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	4,114.4	4,232.1
Corporate | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	27,695.5	23,026.1
Corporate | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	600.5	1,495.8
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	39.6	39.9
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	39.6	18.8
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	0.0	21.1
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
Fixed Maturity Securities
Derivative Assets	27.0	30.4
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	27.0	30.4
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
Fixed Maturity Securities
Derivative Assets	0.5	0.5
Credit Default Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Derivative Assets	0.0	0.0
Credit Default Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Derivative Assets	0.5	0.5
Credit Default Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Derivative Assets	0.0	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	0.0
Private Equity Funds of Funds
Fixed Maturity Securities
Alternative Investments	616.7	483.8
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	616.7	483.8
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	34.6	38.0
Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Liabilities
Derivative Liabilities	22.8	31.1
Derivatives | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Liabilities
Derivative Liabilities	$ 0.0	$ 0.0