Fair Value Measurements by Input Level (Detail) - USD ($) $ in Millions		Mar. 31, 2026	Dec. 31, 2025
Assets, Fair Value Disclosure
Fixed Maturity Securities		$ 33,300.8	$ 33,056.6
Equity Securities		20.0	20.0
Other Long-term Investments		$ 1,664.8	$ 1,670.4
Derivative Asset, Statement of Financial Position [Extensible Enumeration]		Other Long-term Investments	Other Long-term Investments
Derivative Assets		$ 75.5	$ 69.7
Assets, Fair Value Disclosure		$ 34,840.4	$ 34,602.6
Liabilities
Derivative Liability, Statement of Financial Position [Extensible Enumeration]		Other Liabilities	Other Liabilities
Derivative Liabilities		$ 261.0	$ 269.7
Liabilities, Fair Value Disclosure		261.0	269.7
Forwards
Assets, Fair Value Disclosure
Derivative Assets		1.7	2.4
Liabilities
Derivative Liabilities		219.4	223.9
Foreign Exchange Contracts
Assets, Fair Value Disclosure
Derivative Assets		59.0	49.5
Liabilities
Derivative Liabilities		41.6	45.8
Embedded Derivative in Modified Coinsurance Arrangement
Assets, Fair Value Disclosure
Derivative Assets		14.8	17.8
Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure
Other Long-term Investments		1,539.6	1,546.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities		7,920.4	3,959.0
Equity Securities		0.0	0.0
Other Long-term Investments		0.0	0.0
Assets, Fair Value Disclosure		7,920.4	3,959.0
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		25,122.3	28,930.6
Equity Securities		0.2	0.2
Other Long-term Investments		60.9	52.1
Derivative Assets		60.7	51.9
Assets, Fair Value Disclosure		25,183.2	28,982.7
Liabilities
Derivative Liabilities		261.0	269.7
Liabilities, Fair Value Disclosure		261.0	269.7
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Forwards
Assets, Fair Value Disclosure
Derivative Assets		1.7	2.4
Liabilities
Derivative Liabilities		219.4	223.9
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Foreign Exchange Contracts
Assets, Fair Value Disclosure
Derivative Assets		59.0	49.5
Liabilities
Derivative Liabilities		41.6	45.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities		258.1	167.0
Equity Securities		19.8	19.8
Other Long-term Investments		34.6	37.6
Derivative Assets		14.8	17.8
Assets, Fair Value Disclosure		292.7	204.6
Liabilities
Liabilities, Fair Value Disclosure		0.0	0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Forwards
Assets, Fair Value Disclosure
Derivative Assets			0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Embedded Derivative in Modified Coinsurance Arrangement
Assets, Fair Value Disclosure
Derivative Assets		14.8	17.8
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		1,444.1	1,456.3
Assets, Fair Value Disclosure		1,444.1	1,456.3
Liabilities
Liabilities, Fair Value Disclosure		0.0	0.0
United States Government and Government Agencies and Authorities
Assets, Fair Value Disclosure
Fixed Maturity Securities		544.9	545.0
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities		77.7	79.3
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities		467.2	465.7
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		3,000.0	3,058.1
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		3,000.0	3,058.1
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		844.9	870.3
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		844.9	870.3
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		5,005.4	4,933.9
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities		690.3	223.7
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities		4,315.1	4,710.2
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]	1,356.9	1,179.3
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		1,159.2	1,076.2
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities		197.7	103.1
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		22,540.9	22,462.1
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities		7,152.4	3,656.0
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities		15,328.1	18,742.2
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities		60.4	63.9
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	7.9
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	7.9
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		1,444.1	1,456.3
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		$ 1,444.1	$ 1,456.3

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