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Fair Value Measurements by Input Level (Detail) - USD ($)
$ in Millions
Dec. 31, 2022
Dec. 31, 2021
Assets    
Fixed Maturity Securities $ 34,840.8 $ 43,336.0
Derivative Asset, Statement of Financial Position [Extensible Enumeration] Other Long-term Investments  
Derivative Assets $ 89.1 39.5
Equity Securities 25.8 33.7
Other Long-term Investments 1,440.1 1,203.0
Assets, Fair Value Disclosure $ 36,150.0 44,387.8
Derivative Liability, Statement of Financial Position [Extensible Enumeration] Other Liabilities  
Liabilities    
Derivative Liabilities   65.1
Liabilities, Fair Value Disclosure $ 87.9 65.1
Forwards    
Assets    
Derivative Liability, Statement of Financial Position [Extensible Enumeration] Other Liabilities  
Foreign Exchange Contracts    
Assets    
Derivative Liability, Statement of Financial Position [Extensible Enumeration] Other Liabilities  
Liabilities    
Derivative Liabilities   35.0
Embedded Derivative in Modified Coinsurance Arrangement    
Assets    
Derivative Liability, Statement of Financial Position [Extensible Enumeration] Other Liabilities  
Liabilities    
Derivative Liabilities   30.1
Fair Value, Measurements, Recurring    
Assets    
Other Long-term Investments $ 1,309.2 1,051.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets    
Fixed Maturity Securities $ 4,519.1 3,519.5
Derivative Asset, Statement of Financial Position [Extensible Enumeration] Other Long-term Investments  
Derivative Assets   0.0
Equity Securities $ 0.0 0.0
Other Long-term Investments 0.0 0.0
Assets, Fair Value Disclosure 4,519.1 3,519.5
Liabilities    
Liabilities, Fair Value Disclosure $ 0.0 0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Forwards    
Assets    
Derivative Liability, Statement of Financial Position [Extensible Enumeration] Other Liabilities  
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Foreign Exchange Contracts    
Assets    
Derivative Liability, Statement of Financial Position [Extensible Enumeration] Other Liabilities  
Liabilities    
Derivative Liabilities   0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Embedded Derivative in Modified Coinsurance Arrangement    
Assets    
Derivative Liability, Statement of Financial Position [Extensible Enumeration] Other Liabilities  
Liabilities    
Derivative Liabilities   0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets    
Fixed Maturity Securities $ 30,140.8 38,689.1
Derivative Asset, Statement of Financial Position [Extensible Enumeration] Other Long-term Investments  
Derivative Assets   39.5
Equity Securities $ 9.6 27.9
Other Long-term Investments 98.7 67.4
Assets, Fair Value Disclosure 30,239.5 38,756.5
Liabilities    
Liabilities, Fair Value Disclosure $ 74.0 35.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Forwards    
Assets    
Derivative Liability, Statement of Financial Position [Extensible Enumeration] Other Liabilities  
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Foreign Exchange Contracts    
Assets    
Derivative Liability, Statement of Financial Position [Extensible Enumeration] Other Liabilities  
Liabilities    
Derivative Liabilities   35.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Embedded Derivative in Modified Coinsurance Arrangement    
Assets    
Derivative Liability, Statement of Financial Position [Extensible Enumeration] Other Liabilities  
Liabilities    
Derivative Liabilities   0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets    
Fixed Maturity Securities $ 180.9 1,127.4
Derivative Asset, Statement of Financial Position [Extensible Enumeration] Other Long-term Investments  
Derivative Assets   0.0
Equity Securities $ 16.2 5.8
Other Long-term Investments 16.2 5.8
Assets, Fair Value Disclosure 197.1 1,133.2
Liabilities    
Liabilities, Fair Value Disclosure $ 13.9 30.1
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Forwards    
Assets    
Derivative Liability, Statement of Financial Position [Extensible Enumeration] Other Liabilities  
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Foreign Exchange Contracts    
Assets    
Derivative Liability, Statement of Financial Position [Extensible Enumeration] Other Liabilities  
Liabilities    
Derivative Liabilities   0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Embedded Derivative in Modified Coinsurance Arrangement    
Assets    
Derivative Liability, Statement of Financial Position [Extensible Enumeration] Other Liabilities  
Liabilities    
Derivative Liabilities   30.1
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share    
Assets    
Fixed Maturity Securities $ 0.0 0.0
Derivative Asset, Statement of Financial Position [Extensible Enumeration] Other Long-term Investments  
Derivative Assets   0.0
Equity Securities $ 0.0 0.0
Other Long-term Investments 1,194.3 978.6
Assets, Fair Value Disclosure 1,194.3 978.6
Liabilities    
Liabilities, Fair Value Disclosure $ 0.0 0.0
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share | Forwards    
Assets    
Derivative Liability, Statement of Financial Position [Extensible Enumeration] Other Liabilities  
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share | Foreign Exchange Contracts    
Assets    
Derivative Liability, Statement of Financial Position [Extensible Enumeration] Other Liabilities  
Liabilities    
Derivative Liabilities   0.0
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share | Embedded Derivative in Modified Coinsurance Arrangement    
Assets    
Derivative Liability, Statement of Financial Position [Extensible Enumeration] Other Liabilities  
Liabilities    
Derivative Liabilities   0.0
United States Government and Government Agencies and Authorities    
Assets    
Fixed Maturity Securities $ 498.2 580.1
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets    
Fixed Maturity Securities 81.7 0.0
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets    
Fixed Maturity Securities 416.5 580.1
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 3,457.2 4,727.5
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets    
Fixed Maturity Securities 8.7 0.0
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets    
Fixed Maturity Securities 3,448.3 4,714.1
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets    
Fixed Maturity Securities 0.2 13.4
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 827.1 1,146.6
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 827.1 1,125.8
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets    
Fixed Maturity Securities 0.0 20.8
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 4,956.9 6,416.0
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets    
Fixed Maturity Securities 160.1 230.8
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets    
Fixed Maturity Securities 4,796.8 6,140.7
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets    
Fixed Maturity Securities 0.0 44.5
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 573.3 638.3
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 551.3 451.1
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets    
Fixed Maturity Securities 22.0 187.2
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 24,524.6 29,823.4
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets    
Fixed Maturity Securities 4,268.6 3,288.7
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets    
Fixed Maturity Securities 20,097.3 25,673.2
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets    
Fixed Maturity Securities 158.7 861.5
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 3.5 4.1
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 3.5 4.1
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 Asset, Statement of Financial Position [Extensible Enumeration] Other Long-term Investments  
Forwards | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets    
Derivative Asset, Statement of Financial Position [Extensible Enumeration] Other Long-term Investments  
Forwards | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets    
Derivative Asset, Statement of Financial Position [Extensible Enumeration] Other Long-term Investments  
Forwards | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets    
Derivative Asset, Statement of Financial Position [Extensible Enumeration] Other Long-term Investments  
Forwards | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share    
Assets    
Derivative Asset, Statement of Financial Position [Extensible Enumeration] Other Long-term Investments  
Foreign Exchange Contracts    
Assets    
Derivative Asset, Statement of Financial Position [Extensible Enumeration] Other Long-term Investments  
Derivative Assets   39.5
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets    
Derivative Asset, Statement of Financial Position [Extensible Enumeration] Other Long-term Investments  
Derivative Assets   0.0
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets    
Derivative Asset, Statement of Financial Position [Extensible Enumeration] Other Long-term Investments  
Derivative Assets   39.5
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets    
Derivative Asset, Statement of Financial Position [Extensible Enumeration] Other Long-term Investments  
Derivative Assets   0.0
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share    
Assets    
Derivative Asset, Statement of Financial Position [Extensible Enumeration] Other Long-term Investments  
Derivative Assets   0.0
Private Equity Partnerships    
Assets    
Alternative Investments $ 1,194.3 978.6
Private Equity Partnerships | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets    
Alternative Investments 0.0 0.0
Private Equity Partnerships | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets    
Alternative Investments 0.0 0.0
Private Equity Partnerships | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets    
Alternative Investments 0.0 0.0
Private Equity Partnerships | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share    
Assets    
Alternative Investments $ 1,194.3 978.6
Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets    
Derivative Liability, Statement of Financial Position [Extensible Enumeration] Other Liabilities  
Liabilities    
Derivative Liabilities   0.0
Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets    
Derivative Liability, Statement of Financial Position [Extensible Enumeration] Other Liabilities  
Liabilities    
Derivative Liabilities   35.0
Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets    
Derivative Liability, Statement of Financial Position [Extensible Enumeration] Other Liabilities  
Liabilities    
Derivative Liabilities   30.1
Derivatives | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share    
Assets    
Derivative Liability, Statement of Financial Position [Extensible Enumeration] Other Liabilities  
Liabilities    
Derivative Liabilities   $ 0.0