Loss And Loss Adjustment Expense Reserves (Loss And LAE Activity) (Narrative) (Detail) - USD ($) $ in Millions			3 Months Ended		9 Months Ended
	Oct. 18, 2021	Feb. 10, 2021	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Loss And Loss Adjustment Expense Reserves [Line Items]
Weighted average risk-free rate used to discount claim liability					1.52%
Percentage of voters voted by the deadline	27.00%
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			$ 4	$ 8	17	$ 25
Loss and loss adjustment expense reserves			$ 33		$ 33		$ 30
Changes in Loss and LAE Reserves					one-year