Fair Value Measurements by Input Level (Detail) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Assets, Fair Value Disclosure
Fixed Maturity Securities	$ 36,247.8	$ 36,833.9
Equity Securities	31.9	31.9
Other Long-term Investments	$ 1,619.8	$ 1,579.4
Derivative Asset, Statement of Financial Position [Extensible Enumeration]	Other Long-term Investments	Other Long-term Investments
Derivative Assets	$ 100.1	$ 99.9
Assets, Fair Value Disclosure	$ 37,750.1	$ 38,291.9
Liabilities
Derivative Liability, Statement of Financial Position [Extensible Enumeration]	Other Liabilities	Other Liabilities
Derivative Liabilities	$ 147.0	$ 117.7
Liabilities, Fair Value Disclosure	147.0	117.7
Forwards
Liabilities
Derivative Liabilities	113.6	78.0
Foreign Exchange Contracts
Liabilities
Derivative Liabilities	33.4	38.2
Embedded Derivative in Modified Coinsurance Arrangement
Liabilities
Derivative Liabilities		1.5
Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure
Other Long-term Investments	1,502.3	1,458.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities	9,332.2	4,286.5
Equity Securities	0.0	0.0
Other Long-term Investments	0.0	0.0
Assets, Fair Value Disclosure	9,332.2	4,286.5
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	26,786.7	32,391.1
Equity Securities	10.3	10.3
Other Long-term Investments	105.8	110.2
Derivative Assets	95.5	99.9
Assets, Fair Value Disclosure	26,892.5	32,501.3
Liabilities
Liabilities, Fair Value Disclosure	147.0	116.2
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Forwards
Liabilities
Derivative Liabilities	113.6	78.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Foreign Exchange Contracts
Liabilities
Derivative Liabilities	33.4	38.2
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities	128.9	156.3
Equity Securities	21.6	21.6
Other Long-term Investments	26.2	21.6
Derivative Assets	4.6
Assets, Fair Value Disclosure	155.1	177.9
Liabilities
Liabilities, Fair Value Disclosure	0.0	1.5
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Embedded Derivative in Modified Coinsurance Arrangement
Liabilities
Derivative Liabilities		1.5
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	1,370.3	1,326.2
Assets, Fair Value Disclosure	1,370.3	1,326.2
Liabilities
Liabilities, Fair Value Disclosure	0.0	0.0
United States Government and Government Agencies and Authorities
Assets, Fair Value Disclosure
Fixed Maturity Securities	578.2	624.8
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities	79.7	0.0
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities	498.5	624.8
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	3,564.9	3,678.4
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	3,564.9	3,678.4
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	826.8	890.7
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	826.8	890.7
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	5,330.2	5,321.6
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities	776.1	301.3
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities	4,554.1	5,020.3
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	708.7	644.1
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	670.0	611.2
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities	38.7	32.9
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	25,235.4	25,670.7
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities	8,476.4	3,985.2
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities	16,668.8	21,562.1
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities	90.2	123.4
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	3.6	3.6
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	3.6	3.6
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
Forwards
Assets, Fair Value Disclosure
Derivative Assets	36.7	47.5
Forwards | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Derivative Assets	36.7	47.5
Forwards | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Derivative Assets		0.0
Foreign Exchange Contracts
Assets, Fair Value Disclosure
Derivative Assets	58.8	52.4
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Derivative Assets	58.8	52.4
Private Equity Partnerships
Assets, Fair Value Disclosure
Alternative Investments	1,370.3	1,326.2
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	1,370.3	1,326.2
Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Liabilities
Derivative Liabilities	147.0	116.2
Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Liabilities
Derivative Liabilities		$ 1.5
Embedded Derivative in Modified Coinsurance Arrangement
Assets, Fair Value Disclosure
Derivative Assets	4.6
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Derivative Assets	$ 4.6