Fair Value Measurements by Input Level (Detail) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Assets, Fair Value Disclosure
Fixed Maturity Securities	$ 35,374.1	$ 34,840.8
Derivative Assets	Other Long-term Investments	Other Long-term Investments
Equity Securities	$ 27.8	$ 25.8
Other Long-term Investments	1,503.2	1,440.1
Assets, Fair Value Disclosure	$ 36,750.6	$ 36,150.0
Liabilities
Derivative Liabilities	Other Liabilities	Other Liabilities
Liabilities, Fair Value Disclosure	$ 84.8	$ 87.9
Interest Rate Swaps
Liabilities
Derivative Liabilities	Other Liabilities	Other Liabilities
Foreign Exchange Contracts
Liabilities
Derivative Liabilities	Other Liabilities	Other Liabilities
Embedded Derivative in Modified Coinsurance Arrangement
Liabilities
Derivative Liabilities	Other Liabilities	Other Liabilities
Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure
Other Long-term Investments	$ 1,376.5	$ 1,309.2
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities	$ 8,564.7	$ 4,519.1
Derivative Assets	Other Long-term Investments	Other Long-term Investments
Equity Securities	$ 0.0	$ 0.0
Other Long-term Investments	0.0	0.0
Assets, Fair Value Disclosure	8,564.7	4,519.1
Liabilities
Liabilities, Fair Value Disclosure	$ 0.0	$ 0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Interest Rate Swaps
Liabilities
Derivative Liabilities	Other Liabilities	Other Liabilities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Foreign Exchange Contracts
Liabilities
Derivative Liabilities	Other Liabilities	Other Liabilities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Embedded Derivative in Modified Coinsurance Arrangement
Liabilities
Derivative Liabilities	Other Liabilities	Other Liabilities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities	$ 26,656.5	$ 30,140.8
Derivative Assets	Other Long-term Investments	Other Long-term Investments
Equity Securities	$ 10.3	$ 9.6
Other Long-term Investments	90.9	98.7
Assets, Fair Value Disclosure	26,747.4	30,239.5
Liabilities
Liabilities, Fair Value Disclosure	$ 76.4	$ 74.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Interest Rate Swaps
Liabilities
Derivative Liabilities	Other Liabilities	Other Liabilities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Foreign Exchange Contracts
Liabilities
Derivative Liabilities	Other Liabilities	Other Liabilities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Embedded Derivative in Modified Coinsurance Arrangement
Liabilities
Derivative Liabilities	Other Liabilities	Other Liabilities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities	$ 152.9	$ 180.9
Derivative Assets	Other Long-term Investments	Other Long-term Investments
Equity Securities	$ 17.5	$ 16.2
Other Long-term Investments	17.5	16.2
Assets, Fair Value Disclosure	170.4	197.1
Liabilities
Liabilities, Fair Value Disclosure	$ 8.4	$ 13.9
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Interest Rate Swaps
Liabilities
Derivative Liabilities	Other Liabilities	Other Liabilities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Foreign Exchange Contracts
Liabilities
Derivative Liabilities	Other Liabilities	Other Liabilities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Embedded Derivative in Modified Coinsurance Arrangement
Liabilities
Derivative Liabilities	Other Liabilities	Other Liabilities
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	Other Long-term Investments	Other Long-term Investments
Equity Securities	$ 0.0	$ 0.0
Other Long-term Investments	1,268.1	1,194.3
Assets, Fair Value Disclosure	1,268.1	1,194.3
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	Other Liabilities	Other Liabilities
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share | Foreign Exchange Contracts
Liabilities
Derivative Liabilities	Other Liabilities	Other Liabilities
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share | Embedded Derivative in Modified Coinsurance Arrangement
Liabilities
Derivative Liabilities	Other Liabilities	Other Liabilities
United States Government and Government Agencies and Authorities
Assets, Fair Value Disclosure
Fixed Maturity Securities	$ 569.1	$ 498.2
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities	81.7	81.7
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities	487.4	416.5
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	3,537.6	3,457.2
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities	5.0	8.7
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities	3,532.6	3,448.3
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities	0.0	0.2
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	870.7	827.1
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	870.7	827.1
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities	0.0	0.0
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	5,176.2	4,956.9
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities	664.2	160.1
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities	4,512.0	4,796.8
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities	0.0	0.0
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	556.1	573.3
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	532.0	551.3
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities	24.1	22.0
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
All Other Corporate Bonds
Assets, Fair Value Disclosure
Fixed Maturity Securities	24,660.9	24,524.6
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities	7,813.8	4,268.6
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities	16,718.3	20,097.3
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities	128.8	158.7
All Other Corporate Bonds | 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	3.5	3.5
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	3.5	3.5
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
Private Equity Partnerships
Assets, Fair Value Disclosure
Alternative Investments	1,268.1	1,194.3
Private Equity Partnerships | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Alternative Investments	0.0	0.0
Private Equity Partnerships | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Alternative Investments	0.0	0.0
Private Equity Partnerships | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Alternative Investments	0.0	0.0
Private Equity Partnerships | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure
Alternative Investments	$ 1,268.1	$ 1,194.3
Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Liabilities
Derivative Liabilities	Other Liabilities	Other Liabilities
Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Liabilities
Derivative Liabilities	Other Liabilities	Other Liabilities
Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Liabilities
Derivative Liabilities	Other Liabilities	Other Liabilities
Derivatives | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Liabilities
Derivative Liabilities	Other Liabilities	Other Liabilities
Forwards
Assets, Fair Value Disclosure
Derivative Assets	Other Long-term Investments	Other Long-term Investments
Forwards | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Derivative Assets	Other Long-term Investments	Other Long-term Investments
Forwards | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Derivative Assets	Other Long-term Investments	Other Long-term Investments
Forwards | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Derivative Assets	Other Long-term Investments	Other Long-term Investments
Forwards | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure
Derivative Assets	Other Long-term Investments	Other Long-term Investments
Foreign Exchange Contracts
Assets, Fair Value Disclosure
Derivative Assets	Other Long-term Investments	Other Long-term Investments
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Derivative Assets	Other Long-term Investments	Other Long-term Investments
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Derivative Assets	Other Long-term Investments	Other Long-term Investments
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Derivative Assets	Other Long-term Investments	Other Long-term Investments
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure
Derivative Assets	Other Long-term Investments	Other Long-term Investments