Fair Value Measurements by Input Level (Detail) - USD ($) $ in Millions		Jun. 30, 2025	Dec. 31, 2024
Assets, Fair Value Disclosure
Fixed Maturity Securities		$ 35,966.0	$ 35,629.9
Equity Securities		23.9	24.5
Other Long-term Investments		$ 1,659.9	$ 1,694.4
Derivative Asset, Statement of Financial Position [Extensible Enumeration]		Other Long-term Investments	Other Long-term Investments
Derivative Assets		$ 54.4	$ 90.9
Assets, Fair Value Disclosure		$ 37,495.2	$ 37,195.9
Liabilities
Derivative Liability, Statement of Financial Position [Extensible Enumeration]		Other Liabilities	Other Liabilities
Derivative Liabilities		$ 286.5	$ 255.7
Liabilities, Fair Value Disclosure		286.5	255.7
Forwards
Assets, Fair Value Disclosure
Derivative Assets		3.0	6.5
Liabilities
Derivative Liabilities		234.1	223.2
Foreign Exchange Contracts
Assets, Fair Value Disclosure
Derivative Assets		40.8	72.9
Liabilities
Derivative Liabilities		52.4	32.5
Embedded Derivative in Modified Coinsurance Arrangement
Assets, Fair Value Disclosure
Derivative Assets		10.6	11.5
Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure
Other Long-term Investments		1,529.2	1,566.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities		6,221.6	4,180.5
Equity Securities		0.0	0.0
Other Long-term Investments		0.0	0.0
Assets, Fair Value Disclosure		6,221.6	4,180.5
Liabilities
Liabilities, Fair Value Disclosure		0.0	0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities		29,643.1	31,304.4
Equity Securities		0.1	0.1
Other Long-term Investments		43.9	79.5
Derivative Assets		43.8	79.4
Assets, Fair Value Disclosure		29,687.0	31,383.9
Liabilities
Derivative Liabilities		286.5	255.7
Liabilities, Fair Value Disclosure		286.5	255.7
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Forwards
Assets, Fair Value Disclosure
Derivative Assets		3.0	6.5
Liabilities
Derivative Liabilities		234.1	223.2
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Foreign Exchange Contracts
Assets, Fair Value Disclosure
Derivative Assets		40.8	72.9
Liabilities
Derivative Liabilities		52.4	32.5
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities		101.3	145.0
Equity Securities		23.8	24.4
Other Long-term Investments		34.4	35.9
Derivative Assets		10.6	11.5
Assets, Fair Value Disclosure		135.7	180.9
Liabilities
Liabilities, Fair Value Disclosure		0.0	0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Embedded Derivative in Modified Coinsurance Arrangement
Assets, Fair Value Disclosure
Derivative Assets		10.6	11.5
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure
Fixed Maturity Securities		0.0	0.0
Equity Securities		0.0	0.0
Other Long-term Investments		1,450.9	1,450.6
Assets, Fair Value Disclosure		1,450.9	1,450.6
Liabilities
Liabilities, Fair Value Disclosure		0.0	0.0
United States Government and Government Agencies and Authorities
Assets, Fair Value Disclosure
Fixed Maturity Securities		537.8	530.5
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities		0.0	77.9
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities		537.8	452.6
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		3,183.7	3,291.4
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities		0.0	0.0
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities		3,183.7	3,291.4
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities		0.0	0.0
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		873.2	768.1
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		873.2	768.1
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		5,326.7	5,292.9
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities		444.6	174.5
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities		4,882.1	5,118.4
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	[1]	1,093.2	917.2
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities	[2]	0.0	0.0
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities	[2]	1,011.8	843.7
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities	[2]	81.4	73.5
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure
Fixed Maturity Securities	[2]	0.0	0.0
All Other Corporate Bonds
Assets, Fair Value Disclosure
Fixed Maturity Securities		24,943.6	24,822.2
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities		5,777.0	3,928.1
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities		19,146.7	20,822.6
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities		19.9	71.5
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		7.8	7.6
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		7.8	7.6
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		1,450.9	1,450.6
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		$ 1,450.9	$ 1,450.6

[1] 1Includes credit-tranched securities collateralized by loan obligations, auto loans, and other asset types

[2] 1Includes credit-tranched securities collateralized by loan obligations, auto loans, and other asset types