Fair Value Measurements by Input Level (Detail) - USD ($) $ in Millions		Dec. 31, 2025	Dec. 31, 2024
Assets
Fixed Maturity Securities		$ 33,056.6	$ 35,629.9
Derivative Asset, Statement of Financial Position [Extensible Enumeration]		Other Long-term Investments	Other Long-term Investments
Derivative Assets		$ 69.7	$ 90.9
Equity Securities		20.0	24.5
Other Long-term Investments		1,670.4	1,694.4
Assets, Fair Value Disclosure		$ 34,602.6	$ 37,195.9
Liabilities
Derivative Liability, Statement of Financial Position [Extensible Enumeration]		Other Liabilities	Other Liabilities
Derivative Liabilities		$ 269.7	$ 255.7
Liabilities, Fair Value Disclosure		269.7	255.7
Forwards
Liabilities
Derivative Liabilities		223.9	223.2
Embedded Derivative in Modified Coinsurance Arrangement
Assets
Derivative Assets		17.8	11.5
Interest Rate Swaps
Liabilities
Derivative Liabilities		45.8	32.5
Fair Value, Measurements, Recurring
Assets
Other Long-term Investments		1,546.0	1,566.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets
Fixed Maturity Securities		3,959.0	4,180.5
Equity Securities		0.0	0.0
Assets, Fair Value Disclosure		3,959.0	4,180.5
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Embedded Derivative in Modified Coinsurance Arrangement
Assets
Derivative Assets		0.0	0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Fixed Maturity Securities		28,930.6	31,304.4
Derivative Assets		51.9	79.4
Equity Securities		0.2	0.1
Other Long-term Investments		52.1	79.5
Assets, Fair Value Disclosure		28,982.7	31,383.9
Liabilities
Derivative Liabilities		269.7	255.7
Liabilities, Fair Value Disclosure		269.7	255.7
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Forwards
Liabilities
Derivative Liabilities		223.9	223.2
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Embedded Derivative in Modified Coinsurance Arrangement
Assets
Derivative Assets		0.0	0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Interest Rate Swaps
Liabilities
Derivative Liabilities		45.8	32.5
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets
Fixed Maturity Securities		167.0	145.0
Derivative Assets		17.8	11.5
Equity Securities		19.8	24.4
Other Long-term Investments		37.6	35.9
Assets, Fair Value Disclosure		204.6	180.9
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Embedded Derivative in Modified Coinsurance Arrangement
Assets
Derivative Assets		17.8	11.5
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		1,456.3	1,450.6
Assets, Fair Value Disclosure		1,456.3	1,450.6
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share | Embedded Derivative in Modified Coinsurance Arrangement
Assets
Derivative Assets		0.0	0.0
United States Government and Government Agencies and Authorities
Assets
Fixed Maturity Securities		545.0	530.5
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets
Fixed Maturity Securities		79.3	77.9
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Fixed Maturity Securities		465.7	452.6
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		3,058.1	3,291.4
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets
Fixed Maturity Securities		0.0	0.0
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Fixed Maturity Securities		3,058.1	3,291.4
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets
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
Fixed Maturity Securities		0.0	0.0
Foreign Governments
Assets
Fixed Maturity Securities		870.3	768.1
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		870.3	768.1
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		4,933.9	5,292.9
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets
Fixed Maturity Securities		223.7	174.5
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Fixed Maturity Securities		4,710.2	5,118.4
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	[1],[2]	1,179.3	917.2
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets
Fixed Maturity Securities	[2]	0.0	0.0
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Fixed Maturity Securities	[2]	1,076.2	843.7
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets
Fixed Maturity Securities	[2]	103.1	73.5
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets
Fixed Maturity Securities	[2]	0.0	0.0
All Other Corporate Bonds
Assets
Fixed Maturity Securities		22,462.1	24,822.2
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets
Fixed Maturity Securities		3,656.0	3,928.1
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Fixed Maturity Securities		18,742.2	20,822.6
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets
Fixed Maturity Securities		63.9	71.5
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		7.9	7.6
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		7.9	7.6
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		2.4	6.5
Forwards | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Derivative Assets		2.4	6.5
Private Equity Funds
Assets
Alternative Investments		1,456.3	1,450.6
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		1,456.3	1,450.6
Interest Rate Swaps
Assets
Derivative Assets		49.5	72.9
Interest Rate Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Derivative Assets		$ 49.5	$ 72.9

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

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