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Loss And Loss Adjustment Expense Reserves (Loss And LAE Activity) (Narrative) (Detail) - USD ($)
$ in Millions
 3 Months Ended
Feb. 10, 2021
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Loss And Loss Adjustment Expense Reserves [Line Items]        
Weighted average risk-free rate used to discount claim liability   1.56%    
Credit Suisse [Member]        
Loss And Loss Adjustment Expense Reserves [Line Items]        
Damages Sought Value   $ 604    
Damages Paid Value $ 600      
Lae [Member]        
Loss And Loss Adjustment Expense Reserves [Line Items]        
Losses and loss adjustment   12 $ 9  
Loss and loss adjustment expense reserves   $ 39   $ 30
Changes in Loss and LAE Reserves   one-year