Fair Value Measurements by Input Level (Detail) - USD ($) $ in Millions		Dec. 31, 2024	Dec. 31, 2023
Assets
Fixed Maturity Securities		$ 35,629.9	$ 36,833.9
Derivative Asset, Statement of Financial Position [Extensible Enumeration]		Other Long-term Investments	Other Long-term Investments
Derivative Assets		$ 90.9	$ 99.9
Equity Securities		24.5	31.9
Other Long-term Investments		1,694.4	1,579.4
Assets, Fair Value Disclosure		$ 37,195.9	$ 38,291.9
Liabilities
Derivative Liability, Statement of Financial Position [Extensible Enumeration]		Other Liabilities	Other Liabilities
Derivative Liabilities		$ 255.7	$ 117.7
Liabilities, Fair Value Disclosure		255.7	117.7
Forwards
Liabilities
Derivative Liabilities		223.2	78.0
Foreign Exchange Contracts
Liabilities
Derivative Liabilities		32.5	38.2
Embedded Derivative in Modified Coinsurance Arrangement
Assets
Derivative Assets		11.5
Liabilities
Derivative Liabilities			1.5
Fair Value, Measurements, Recurring
Assets
Other Long-term Investments		1,566.0	1,458.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets
Fixed Maturity Securities		4,180.5	4,286.5
Equity Securities		0.0	0.0
Assets, Fair Value Disclosure		4,180.5	4,286.5
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Embedded Derivative in Modified Coinsurance Arrangement
Assets
Derivative Assets		0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Fixed Maturity Securities		31,304.4	32,391.1
Derivative Assets		79.4	99.9
Equity Securities		0.1	10.3
Other Long-term Investments		79.5	110.2
Assets, Fair Value Disclosure		31,383.9	32,501.3
Liabilities
Derivative Liabilities		255.7	116.2
Liabilities, Fair Value Disclosure		255.7	116.2
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Forwards
Liabilities
Derivative Liabilities		223.2	78.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Foreign Exchange Contracts
Liabilities
Derivative Liabilities		32.5	38.2
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Embedded Derivative in Modified Coinsurance Arrangement
Assets
Derivative Assets		0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets
Fixed Maturity Securities		145.0	156.3
Derivative Assets		11.5
Equity Securities		24.4	21.6
Other Long-term Investments		35.9	21.6
Assets, Fair Value Disclosure		180.9	177.9
Liabilities
Derivative Liabilities			1.5
Liabilities, Fair Value Disclosure			1.5
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Embedded Derivative in Modified Coinsurance Arrangement
Assets
Derivative Assets		11.5
Liabilities
Derivative Liabilities			1.5
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		1,450.6	1,326.2
Assets, Fair Value Disclosure		1,450.6	1,326.2
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share | Embedded Derivative in Modified Coinsurance Arrangement
Assets
Derivative Assets		0.0
United States Government and Government Agencies and Authorities
Assets
Fixed Maturity Securities		530.5	624.8
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets
Fixed Maturity Securities		77.9	0.0
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Fixed Maturity Securities		452.6	624.8
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		3,291.4	3,678.4
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		3,291.4	3,678.4
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		768.1	890.7
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		768.1	890.7
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		5,292.9	5,321.6
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets
Fixed Maturity Securities		174.5	301.3
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Fixed Maturity Securities		5,118.4	5,020.3
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]	917.2	644.1
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		843.7	611.2
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets
Fixed Maturity Securities		73.5	32.9
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		24,822.2	25,670.7
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets
Fixed Maturity Securities		3,928.1	3,985.2
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Fixed Maturity Securities		20,822.6	21,562.1
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets
Fixed Maturity Securities		71.5	123.4
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		7.6	3.6
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		7.6	3.6
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		6.5	47.5
Forwards | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Derivative Assets		6.5	47.5
Foreign Exchange Contracts
Assets
Derivative Assets		72.9	52.4
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets
Derivative Assets		72.9	52.4
Private Equity Funds
Assets
Alternative Investments		1,450.6	1,326.2
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		$ 1,450.6	$ 1,326.2

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