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Fair Value Measurements by Input Level (Detail) - USD ($)
$ in Millions
Sep. 30, 2023
Dec. 31, 2022
Assets, Fair Value Disclosure    
Fixed Maturity Securities $ 34,070.7 $ 34,840.8
Derivative Assets Other Long-term Investments Other Long-term Investments
Equity Securities $ 27.9 $ 25.8
Other Long-term Investments 1,506.9 1,440.1
Assets, Fair Value Disclosure $ 35,455.2 $ 36,150.0
Liabilities    
Derivative Liabilities Other Liabilities Other Liabilities
Liabilities, Fair Value Disclosure $ 212.7 $ 87.9
Forwards    
Liabilities    
Derivative Liabilities Other Liabilities Other Liabilities
Foreign Exchange Contracts    
Liabilities    
Derivative Liabilities Other Liabilities Other Liabilities
Embedded Derivative in Modified Coinsurance Arrangement    
Liabilities    
Derivative Liabilities   Other Liabilities
Fair Value, Measurements, Recurring    
Assets, Fair Value Disclosure    
Other Long-term Investments $ 1,384.5 $ 1,309.2
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets, Fair Value Disclosure    
Fixed Maturity Securities $ 8,078.6 $ 4,519.1
Derivative Assets Other Long-term Investments Other Long-term Investments
Equity Securities $ 0.0 $ 0.0
Other Long-term Investments 0.0 0.0
Assets, Fair Value Disclosure 8,078.6 4,519.1
Liabilities    
Liabilities, Fair Value Disclosure $ 0.0 $ 0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Forwards    
Liabilities    
Derivative Liabilities Other Liabilities Other Liabilities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Foreign Exchange Contracts    
Liabilities    
Derivative Liabilities Other Liabilities Other Liabilities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Embedded Derivative in Modified Coinsurance Arrangement    
Liabilities    
Derivative Liabilities   Other Liabilities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets, Fair Value Disclosure    
Fixed Maturity Securities $ 25,838.6 $ 30,140.8
Derivative Assets Other Long-term Investments Other Long-term Investments
Equity Securities $ 10.4 $ 9.6
Other Long-term Investments 85.2 98.7
Assets, Fair Value Disclosure 25,923.8 30,239.5
Liabilities    
Liabilities, Fair Value Disclosure $ 212.7 $ 74.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Forwards    
Liabilities    
Derivative Liabilities Other Liabilities Other Liabilities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Foreign Exchange Contracts    
Liabilities    
Derivative Liabilities Other Liabilities Other Liabilities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Embedded Derivative in Modified Coinsurance Arrangement    
Liabilities    
Derivative Liabilities   Other Liabilities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets, Fair Value Disclosure    
Fixed Maturity Securities $ 153.5 $ 180.9
Derivative Assets Other Long-term Investments Other Long-term Investments
Equity Securities $ 17.5 $ 16.2
Other Long-term Investments 18.4 16.2
Assets, Fair Value Disclosure 171.9 197.1
Liabilities    
Liabilities, Fair Value Disclosure $ 0.0 $ 13.9
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Forwards    
Liabilities    
Derivative Liabilities Other Liabilities Other Liabilities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Foreign Exchange Contracts    
Liabilities    
Derivative Liabilities Other Liabilities Other Liabilities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Embedded Derivative in Modified Coinsurance Arrangement    
Liabilities    
Derivative Liabilities   Other Liabilities
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share    
Assets, Fair Value Disclosure    
Fixed Maturity Securities $ 0.0 $ 0.0
Derivative Assets Other Long-term Investments Other Long-term Investments
Equity Securities $ 0.0 $ 0.0
Other Long-term Investments 1,280.9 1,194.3
Assets, Fair Value Disclosure 1,280.9 1,194.3
Liabilities    
Liabilities, Fair Value Disclosure $ 0.0 $ 0.0
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share | Forwards    
Liabilities    
Derivative Liabilities Other Liabilities Other Liabilities
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share | Foreign Exchange Contracts    
Liabilities    
Derivative Liabilities Other Liabilities Other Liabilities
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share | Embedded Derivative in Modified Coinsurance Arrangement    
Liabilities    
Derivative Liabilities   Other Liabilities
United States Government and Government Agencies and Authorities    
Assets, Fair Value Disclosure    
Fixed Maturity Securities $ 575.0 $ 498.2
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets, Fair Value Disclosure    
Fixed Maturity Securities 77.0 81.7
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets, Fair Value Disclosure    
Fixed Maturity Securities 498.0 416.5
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 3,391.1 3,457.2
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets, Fair Value Disclosure    
Fixed Maturity Securities 0.0 8.7
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets, Fair Value Disclosure    
Fixed Maturity Securities 3,391.1 3,448.3
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets, Fair Value Disclosure    
Fixed Maturity Securities 0.0 0.2
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 819.6 827.1
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 819.6 827.1
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 4,961.2 4,956.9
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets, Fair Value Disclosure    
Fixed Maturity Securities 650.6 160.1
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets, Fair Value Disclosure    
Fixed Maturity Securities 4,310.6 4,796.8
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 604.9 573.3
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 559.7 551.3
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets, Fair Value Disclosure    
Fixed Maturity Securities 45.2 22.0
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 23,715.4 24,524.6
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets, Fair Value Disclosure    
Fixed Maturity Securities 7,351.0 4,268.6
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets, Fair Value Disclosure    
Fixed Maturity Securities 16,256.1 20,097.3
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets, Fair Value Disclosure    
Fixed Maturity Securities 108.3 158.7
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 3.5 3.5
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 3.5 3.5
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 Other Long-term Investments Other Long-term Investments
Forwards | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets, Fair Value Disclosure    
Derivative Assets Other Long-term Investments Other Long-term Investments
Forwards | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets, Fair Value Disclosure    
Derivative Assets Other Long-term Investments Other Long-term Investments
Forwards | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets, Fair Value Disclosure    
Derivative Assets Other Long-term Investments Other Long-term Investments
Forwards | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share    
Assets, Fair Value Disclosure    
Derivative Assets Other Long-term Investments Other Long-term Investments
Foreign Exchange Contracts    
Assets, Fair Value Disclosure    
Derivative Assets Other Long-term Investments Other Long-term Investments
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets, Fair Value Disclosure    
Derivative Assets Other Long-term Investments Other Long-term Investments
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets, Fair Value Disclosure    
Derivative Assets Other Long-term Investments Other Long-term Investments
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets, Fair Value Disclosure    
Derivative Assets Other Long-term Investments Other Long-term Investments
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share    
Assets, Fair Value Disclosure    
Derivative Assets Other Long-term Investments Other Long-term Investments
Embedded Derivative in Modified Coinsurance Arrangement    
Assets, Fair Value Disclosure    
Derivative Assets Other Long-term Investments  
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets, Fair Value Disclosure    
Derivative Assets Other Long-term Investments  
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets, Fair Value Disclosure    
Derivative Assets Other Long-term Investments  
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets, Fair Value Disclosure    
Derivative Assets Other Long-term Investments  
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share    
Assets, Fair Value Disclosure    
Derivative Assets Other Long-term Investments  
Private Equity Partnerships    
Assets, Fair Value Disclosure    
Alternative Investments $ 1,280.9 $ 1,194.3
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 $ 1,280.9 $ 1,194.3
Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Liabilities    
Derivative Liabilities Other Liabilities Other Liabilities
Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Liabilities    
Derivative Liabilities Other Liabilities Other Liabilities
Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Liabilities    
Derivative Liabilities Other Liabilities Other Liabilities
Derivatives | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share    
Liabilities    
Derivative Liabilities Other Liabilities Other Liabilities