Fair Value Measurements by Input Level (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Fixed Maturity Securities
Fixed Maturity Securities	$ 49,270.2	$ 47,443.7
Derivative Assets	38.3	27.5
Equity Securities	24.1	32.6
Other Long-term Investments	931.0	844.2
Assets, Fair Value Disclosure	50,021.2	48,120.5
Liabilities
Derivative Liabilities	87.7	57.4
Liabilities, Fair Value Disclosure	87.7	57.4
Interest Rate Swaps
Liabilities
Derivative Liabilities		0.6
Foreign Exchange Contracts
Liabilities
Derivative Liabilities	34.0	34.0
Embedded Derivative in Modified Coinsurance Arrangement
Liabilities
Derivative Liabilities	53.7	22.8
Fair Value, Measurements, Recurring
Fixed Maturity Securities
Other Long-term Investments	751.0	676.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	9,124.8	4,698.3
Derivative Assets	0.0	0.0
Equity Securities	19.3	0.0
Other Long-term Investments	19.3	0.0
Assets, Fair Value Disclosure	9,144.1	4,698.3
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
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	38,959.3	42,032.6
Derivative Assets	38.3	27.5
Equity Securities	0.2	28.0
Other Long-term Investments	38.5	55.5
Assets, Fair Value Disclosure	38,997.8	42,088.1
Liabilities
Liabilities, Fair Value Disclosure	34.0	34.6
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Interest Rate Swaps
Liabilities
Derivative Liabilities		0.6
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Foreign Exchange Contracts
Liabilities
Derivative Liabilities	34.0	34.0
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	1,186.1	712.8
Derivative Assets	0.0	0.0
Equity Securities	4.6	4.6
Other Long-term Investments	4.6	4.6
Assets, Fair Value Disclosure	1,190.7	717.4
Liabilities
Liabilities, Fair Value Disclosure	53.7	22.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Interest Rate Swaps
Liabilities
Derivative Liabilities		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	53.7	22.8
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	688.6	616.7
Assets, Fair Value Disclosure	688.6	616.7
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
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
United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	1,249.9	1,401.7
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	0.0	412.8
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	1,249.9	988.9
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Fixed Maturity Securities	3,930.0	3,363.4
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	18.5	0.0
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	3,897.1	3,321.6
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	14.4	41.8
States, Municipalities, and Political Subdivisions | 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	1,115.1	1,017.7
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	1,093.4	995.9
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	21.7	21.8
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	8,016.0	7,732.2
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	550.0	171.1
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	7,104.2	7,546.5
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	361.8	14.6
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	1,432.1	1,478.7
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	1,344.3	1,444.6
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	87.8	34.1
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 Debt Securities
Fixed Maturity Securities
Fixed Maturity Securities	33,517.6	32,410.4
Corporate Debt Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	8,556.3	4,114.4
Corporate Debt Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	24,260.9	27,695.5
Corporate Debt Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	700.4	600.5
Corporate Debt Securities | 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	9.5	39.6
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	9.5	39.6
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
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
Forwards
Fixed Maturity Securities
Derivative Assets	0.1
Forwards | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Derivative Assets	0.0
Forwards | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Derivative Assets	0.1
Forwards | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Derivative Assets	0.0
Forwards | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fixed Maturity Securities
Derivative Assets	0.0
Foreign Exchange Contracts
Fixed Maturity Securities
Derivative Assets	38.0	27.0
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	38.0	27.0
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	0.2	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	0.2	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
Fixed Maturity Securities
Alternative Investments	688.6	616.7
Private Equity Funds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed Maturity Securities
Alternative Investments	0.0	0.0
Private Equity Funds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fixed Maturity Securities
Alternative Investments	0.0	0.0
Private Equity Funds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fixed Maturity Securities
Alternative Investments	0.0	0.0
Private Equity Funds | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fixed Maturity Securities
Alternative Investments	688.6	616.7
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	34.0	34.6
Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Liabilities
Derivative Liabilities	53.7	22.8
Derivatives | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Liabilities
Derivative Liabilities	$ 0.0	$ 0.0