Fair Value Measurements by Input Level (Detail) - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021
Assets, Fair Value Disclosure
Fixed Maturity Securities	$ 36,576.4	$ 43,336.0
Derivative Assets	93.5	39.5
Equity Securities	37.0	33.7
Other Long-term Investments	1,391.6	1,203.0
Assets, Fair Value Disclosure	37,826.9	44,387.8
Liabilities
Derivative Liabilities	64.6	65.1
Liabilities, Fair Value Disclosure	64.6	65.1
Foreign Exchange Contracts
Liabilities
Derivative Liabilities	27.5	35.0
Embedded Derivative in Modified Coinsurance Arrangement
Liabilities
Derivative Liabilities	34.3	30.1
Interest Rate Swaps
Liabilities
Derivative Liabilities	2.8
Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure
Other Long-term Investments	1,250.5	1,051.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities	7,378.1	3,519.5
Derivative Assets	0.0	0.0
Equity Securities	0.0
Other Long-term Investments	0.0	0.0
Assets, Fair Value Disclosure	7,378.1	3,519.5
Liabilities
Liabilities, Fair Value Disclosure	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 1 | Interest Rate Swaps
Liabilities
Derivative Liabilities	0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities	29,049.4	38,689.1
Derivative Assets	93.5	39.5
Equity Securities	26.1	27.9
Other Long-term Investments	119.6	67.4
Assets, Fair Value Disclosure	29,169.0	38,756.5
Liabilities
Liabilities, Fair Value Disclosure	30.3	35.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Foreign Exchange Contracts
Liabilities
Derivative Liabilities	27.5	35.0
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 2 | Interest Rate Swaps
Liabilities
Derivative Liabilities	2.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities	148.9	1,127.4
Derivative Assets	0.0	0.0
Equity Securities	10.9	5.8
Other Long-term Investments	10.9	5.8
Assets, Fair Value Disclosure	159.8	1,133.2
Liabilities
Liabilities, Fair Value Disclosure	34.3	30.1
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	34.3	30.1
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Interest Rate Swaps
Liabilities
Derivative Liabilities	0.0
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure
Fixed Maturity Securities	0.0	0.0
Derivative Assets	0.0	0.0
Equity Securities	0.0	0.0
Other Long-term Investments	1,120.0	978.6
Assets, Fair Value Disclosure	1,120.0	978.6
Liabilities
Liabilities, Fair Value Disclosure	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
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share | Interest Rate Swaps
Liabilities
Derivative Liabilities	0.0
United States Government and Government Agencies and Authorities
Assets, Fair Value Disclosure
Fixed Maturity Securities	504.7	580.1
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities	87.0	0.0
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities	417.7	580.1
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
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
Assets, Fair Value Disclosure
Fixed Maturity Securities	0.0	0.0
States, Municipalities, and Political Subdivisions
Assets, Fair Value Disclosure
Fixed Maturity Securities	3,930.4	4,727.5
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities	9.2	0.0
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities	3,921.2	4,714.1
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities	0.0	13.4
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure
Fixed Maturity Securities	0.0	0.0
Foreign Governments
Assets, Fair Value Disclosure
Fixed Maturity Securities	901.4	1,146.6
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities	0.0	0.0
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities	881.2	1,125.8
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities	20.2	20.8
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure
Fixed Maturity Securities	0.0	0.0
Public Utilities
Assets, Fair Value Disclosure
Fixed Maturity Securities	5,388.4	6,416.0
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities	290.5	230.8
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities	5,084.3	6,140.7
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities	13.6	44.5
Public Utilities | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure
Fixed Maturity Securities	0.0	0.0
Mortgage/Asset-backed Securities
Assets, Fair Value Disclosure
Fixed Maturity Securities	559.9	638.3
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities	0.0	0.0
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities	548.3	451.1
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities	11.6	187.2
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure
Fixed Maturity Securities	0.0	0.0
Corporate Debt Securities
Assets, Fair Value Disclosure
Fixed Maturity Securities	25,287.9	29,823.4
Corporate Debt Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities	6,991.4	3,288.7
Corporate Debt Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities	18,193.0	25,673.2
Corporate Debt Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities	103.5	861.5
Corporate Debt Securities | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure
Fixed Maturity Securities	0.0	0.0
Redeemable Preferred Stocks
Assets, Fair Value Disclosure
Fixed Maturity Securities	3.7	4.1
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities	0.0	0.0
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities	3.7	4.1
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities	0.0	0.0
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure
Fixed Maturity Securities	0.0	0.0
Forwards
Assets, Fair Value Disclosure
Derivative Assets	5.1
Forwards | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Derivative Assets	0.0
Forwards | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Derivative Assets	5.1
Forwards | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Derivative Assets	0.0
Forwards | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure
Derivative Assets	0.0
Foreign Exchange Contracts
Assets, Fair Value Disclosure
Derivative Assets	88.4	39.5
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Derivative Assets	0.0	0.0
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Derivative Assets	88.4	39.5
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Derivative Assets	0.0	0.0
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure
Derivative Assets	0.0	0.0
Private Equity Funds
Assets, Fair Value Disclosure
Alternative Investments	1,120.0	978.6
Private Equity Funds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Alternative Investments	0.0	0.0
Private Equity Funds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Alternative Investments	0.0	0.0
Private Equity Funds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Alternative Investments	0.0	0.0
Private Equity Funds | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure
Alternative Investments	1,120.0	978.6
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	30.3	35.0
Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Liabilities
Derivative Liabilities	34.3	30.1
Derivatives | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Liabilities
Derivative Liabilities	$ 0.0	$ 0.0