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Investments - Summary of Aggregate Contractual or Notional Amounts and Estimated Fair Values Related to Derivative Financial Instruments (Detail) - USD ($)
$ in Millions
Jun. 30, 2019
Dec. 31, 2018
Embedded Derivative on Funds Withheld Liability [Member]    
Derivative [Line Items]    
Contractual/Notional Amount   $ 172
Estimated Fair Value, Asset   4
With Hedge Designation [Member] | Interest Rate Swaps [Member]    
Derivative [Line Items]    
Contractual/Notional Amount $ 540 500
Estimated Fair Value, Asset   11
Estimated Fair Value, (Liability) (9)  
Without Hedge Designation [Member] | Equity Markets, Options - Purchased [Member]    
Derivative [Line Items]    
Contractual/Notional Amount 303 213
Estimated Fair Value, Asset 5 18
Without Hedge Designation [Member] | Equity Markets, Options - Written [Member]    
Derivative [Line Items]    
Contractual/Notional Amount 95 239
Estimated Fair Value, (Liability) (3) (17)
Without Hedge Designation [Member] | Commodity Futures [Member]    
Derivative [Line Items]    
Contractual/Notional Amount 11  
Without Hedge Designation [Member] | Commodity Futures [Member] | Long [Member]    
Derivative [Line Items]    
Contractual/Notional Amount   $ 32
Without Hedge Designation [Member] | Embedded Derivative on Funds Withheld Liability [Member]    
Derivative [Line Items]    
Contractual/Notional Amount 172  
Estimated Fair Value, (Liability) $ (8)