Fair Value Measurements by Input Level (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Assets
Fixed Maturity Securities	$ 36,833.9	$ 34,840.8
Derivative Asset, Statement of Financial Position [Extensible Enumeration]	Other Assets	Other Assets
Derivative Assets	$ 99.9	$ 89.1
Equity Securities	31.9	25.8
Other Long-term Investments	1,579.4	1,440.1
Assets, Fair Value Disclosure	$ 38,291.9	$ 36,150.0
Derivative Liability, Statement of Financial Position [Extensible Enumeration]	Other Liabilities	Other Liabilities
Derivative Liabilities	$ 117.7	$ 87.9
Liabilities
Liabilities, Fair Value Disclosure	117.7	87.9
Forwards
Assets
Derivative Liabilities	78.0	48.5
Foreign Exchange Contracts
Assets
Derivative Liabilities	38.2	25.5
Embedded Derivative in Modified Coinsurance Arrangement
Assets
Derivative Liabilities	1.5	13.9
Fair Value, Measurements, Recurring
Assets
Other Long-term Investments	1,458.0	1,309.2
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets
Fixed Maturity Securities	4,286.5	4,519.1
Equity Securities	0.0	0.0
Assets, Fair Value Disclosure	4,286.5	4,519.1
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Fixed Maturity Securities	32,391.1	30,140.8
Derivative Assets	99.9	89.1
Equity Securities	10.3	9.6
Other Long-term Investments	110.2	98.7
Assets, Fair Value Disclosure	32,501.3	30,239.5
Derivative Liabilities	116.2	74.0
Liabilities
Liabilities, Fair Value Disclosure	116.2	74.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Forwards
Assets
Derivative Liabilities	78.0	48.5
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Foreign Exchange Contracts
Assets
Derivative Liabilities	38.2	25.5
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets
Fixed Maturity Securities	156.3	180.9
Equity Securities	21.6	16.2
Other Long-term Investments	21.6	16.2
Assets, Fair Value Disclosure	177.9	197.1
Derivative Liabilities	1.5	13.9
Liabilities
Liabilities, Fair Value Disclosure	1.5	13.9
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Embedded Derivative in Modified Coinsurance Arrangement
Assets
Derivative Liabilities	1.5	13.9
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets
Fixed Maturity Securities	0.0	0.0
Equity Securities	0.0	0.0
Other Long-term Investments	1,326.2	1,194.3
Assets, Fair Value Disclosure	1,326.2	1,194.3
United States Government and Government Agencies and Authorities
Assets
Fixed Maturity Securities	624.8	498.2
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets
Fixed Maturity Securities	0.0	81.7
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Fixed Maturity Securities	624.8	416.5
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets
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
Fixed Maturity Securities	0.0	0.0
States, Municipalities, and Political Subdivisions
Assets
Fixed Maturity Securities	3,678.4	3,457.2
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets
Fixed Maturity Securities	0.0	8.7
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Fixed Maturity Securities	3,678.4	3,448.3
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets
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
Fixed Maturity Securities	0.0	0.0
Foreign Governments
Assets
Fixed Maturity Securities	890.7	827.1
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets
Fixed Maturity Securities	0.0	0.0
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Fixed Maturity Securities	890.7	827.1
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets
Fixed Maturity Securities	0.0	0.0
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets
Fixed Maturity Securities	0.0	0.0
Public Utilities
Assets
Fixed Maturity Securities	5,321.6	4,956.9
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets
Fixed Maturity Securities	301.3	160.1
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Fixed Maturity Securities	5,020.3	4,796.8
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets
Fixed Maturity Securities	0.0	0.0
Public Utilities | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets
Fixed Maturity Securities	0.0	0.0
Mortgage/Asset-backed Securities
Assets
Fixed Maturity Securities	644.1	573.3
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets
Fixed Maturity Securities	0.0	0.0
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Fixed Maturity Securities	611.2	551.3
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets
Fixed Maturity Securities	32.9	22.0
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets
Fixed Maturity Securities	0.0	0.0
All Other Corporate Bonds
Assets
Fixed Maturity Securities	25,670.7	24,524.6
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets
Fixed Maturity Securities	3,985.2	4,268.6
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Fixed Maturity Securities	21,562.1	20,097.3
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets
Fixed Maturity Securities	123.4	158.7
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets
Fixed Maturity Securities	0.0	0.0
Redeemable Preferred Stocks
Assets
Fixed Maturity Securities	3.6	3.5
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets
Fixed Maturity Securities	0.0	0.0
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Fixed Maturity Securities	3.6	3.5
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets
Fixed Maturity Securities	0.0	0.0
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets
Fixed Maturity Securities	0.0	0.0
Forwards
Assets
Derivative Assets	47.5	5.6
Forwards | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Derivative Assets	47.5	5.6
Foreign Exchange Contracts
Assets
Derivative Assets	52.4	83.5
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Derivative Assets	52.4	83.5
Private Equity Funds
Assets
Alternative Investments	1,326.2	1,194.3
Private Equity Funds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets
Alternative Investments	0.0	0.0
Private Equity Funds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Alternative Investments	0.0	0.0
Private Equity Funds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets
Alternative Investments	0.0	0.0
Private Equity Funds | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets
Alternative Investments	$ 1,326.2	$ 1,194.3