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Fair Value Measurement, Recurring (Detail) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities $ 44,500.1  
Derivative Asset 21.5 $ 19.5
Investments Net Asset Value 413.3  
Other Long-term Investments 706.5 646.8
Fair Value, Measurements, Recurring    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 44,500.1 45,457.8
Marketable Securities, Equity Securities 72.4 11.7
Other Long-term Investments 507.2 438.4
Assets, Fair Value Disclosure, Recurring 45,007.3 45,896.2
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability 69.0 68.1
Liabilities, Fair Value Disclosure, Recurring 69.0 68.1
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 9,196.6 4,170.4
Marketable Securities, Equity Securities 24.9 0.2
Other Long-term Investments 24.9 0.2
Assets, Fair Value Disclosure, Recurring 9,221.5 4,170.6
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability 0.0 0.0
Liabilities, Fair Value Disclosure, Recurring 0.0 0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 33,941.4 39,906.8
Marketable Securities, Equity Securities 46.4 10.4
Other Long-term Investments 67.9 29.9
Assets, Fair Value Disclosure, Recurring 34,009.3 39,936.7
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability 51.4 52.2
Liabilities, Fair Value Disclosure, Recurring 51.4 52.2
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 1,362.1 1,380.6
Marketable Securities, Equity Securities 1.1 1.1
Other Long-term Investments 1.1 1.1
Assets, Fair Value Disclosure, Recurring 1,363.2 1,381.7
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability 17.6 15.9
Liabilities, Fair Value Disclosure, Recurring 17.6 15.9
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 0.0 0.0
Marketable Securities, Equity Securities 0.0 0.0
Other Long-term Investments 413.3 407.2
Assets, Fair Value Disclosure, Recurring 413.3 407.2
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability 0.0 0.0
Liabilities, Fair Value Disclosure, Recurring 0.0 0.0
Foreign Exchange Contracts | Fair Value, Measurements, Recurring    
Assets, Fair Value Disclosure [Abstract]    
Derivative Asset 21.5 19.5
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability 44.2 46.9
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets, Fair Value Disclosure [Abstract]    
Derivative Asset 0.0 0.0
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability 0.0 0.0
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets, Fair Value Disclosure [Abstract]    
Derivative Asset 21.5 19.5
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability 44.2 46.9
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets, Fair Value Disclosure [Abstract]    
Derivative Asset 0.0 0.0
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability 0.0 0.0
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share    
Assets, Fair Value Disclosure [Abstract]    
Derivative Asset 0.0 0.0
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability 0.0 0.0
Interest Rate Swaps | Fair Value, Measurements, Recurring    
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability 7.2 5.1
Interest Rate Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability 0.0 0.0
Interest Rate Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability 7.2 5.1
Interest Rate Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability 0.0 0.0
Interest Rate Swaps | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share    
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability 0.0 0.0
Credit Default Swaps | Fair Value, Measurements, Recurring    
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability   0.2
Credit Default Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability   0.0
Credit Default Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability   0.2
Credit Default Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability   0.0
Credit Default Swaps | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share    
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability   0.0
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring    
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability 17.6 15.9
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability 0.0 0.0
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability 0.0 0.0
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability 17.6 15.9
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share    
Liabilities, Fair Value Disclosure [Abstract]    
Derivative Liability 0.0 0.0
Equity Method Investments | Fair Value, Measurements, Recurring    
Assets, Fair Value Disclosure [Abstract]    
Investments Net Asset Value 413.3 407.2
Equity Method Investments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets, Fair Value Disclosure [Abstract]    
Investments Net Asset Value 0.0 0.0
Equity Method Investments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets, Fair Value Disclosure [Abstract]    
Investments Net Asset Value 0.0 0.0
Equity Method Investments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets, Fair Value Disclosure [Abstract]    
Investments Net Asset Value 0.0 0.0
Equity Method Investments | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share    
Assets, Fair Value Disclosure [Abstract]    
Investments Net Asset Value 413.3 407.2
United States Government and Government Agencies and Authorities    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 1,546.3 1,482.5
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 1,546.3 1,482.5
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 495.3 460.1
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 1,051.0 1,022.4
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets, Fair Value Disclosure [Abstract]    
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 [Abstract]    
Fixed Maturity Securities 0.0 0.0
States, Municipalities, and Political Subdivisions    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 2,357.3 2,336.9
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 2,357.3 2,336.9
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets, Fair Value Disclosure [Abstract]    
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 [Abstract]    
Fixed Maturity Securities 2,321.6 2,336.9
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 35.7 0.0
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 0.0 0.0
Foreign Governments    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 873.7 863.9
Foreign Governments | Fair Value, Measurements, Recurring    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 873.7 863.9
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 0.0 0.0
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 841.4 863.9
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 32.3 0.0
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 0.0 0.0
Public Utilities    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 7,981.7 8,236.5
Public Utilities | Fair Value, Measurements, Recurring    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 7,981.7 8,236.5
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 574.1 154.2
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 7,123.4 7,874.6
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 284.2 207.7
Public Utilities | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 0.0 0.0
Mortgage/Asset-Backed Securities    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 1,842.8 1,973.6
Mortgage/Asset-Backed Securities | Fair Value, Measurements, Recurring    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 1,842.8 1,973.6
Mortgage/Asset-Backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 0.0 0.0
Mortgage/Asset-Backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 1,842.3 1,973.6
Mortgage/Asset-Backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 0.5 0.0
Mortgage/Asset-Backed Securities | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 0.0 0.0
All Other Corporate Bonds | Fair Value, Measurements, Recurring    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 29,857.0 30,522.4
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 8,127.2 3,556.1
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 20,742.6 25,816.2
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 987.2 1,150.1
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 0.0 0.0
Redeemable Preferred Stocks    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 41.3 42.0
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 41.3 42.0
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 0.0 0.0
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 19.1 19.2
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities 22.2 22.8
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share    
Assets, Fair Value Disclosure [Abstract]    
Fixed Maturity Securities $ 0.0 $ 0.0