Loss And Loss Adjustment Expense Reserves (Loss And LAE Activity) (Narrative) (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
	Feb. 10, 2021	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Loss And Loss Adjustment Expense Reserves [Line Items]
Weighted average risk-free rate used to discount claim liability				1.49%
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		$ 1	$ 8	13	$ 17
Loss and loss adjustment expense reserves		$ 35		$ 35		$ 30
Changes in Loss and LAE Reserves				one-year