Loss And Loss Adjustment Expense Reserves (Loss And LAE Activity) (Narrative) (Detail) - USD ($) $ in Millions			1 Months Ended	12 Months Ended
	Feb. 10, 2021	Jan. 31, 2021	Jan. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Loss And Loss Adjustment Expense Reserves [Line Items]
Weighted average risk-free rate used to discount claim liability				0.92%
Subsequent Event [Member] | Credit Suisse [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Damages Sought Value		$ 604	$ 604
Damages Paid Value	$ 600
Lae [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Losses and loss adjustment				$ 51	$ 29	$ 23
Loss and loss adjustment expense reserves				$ 30	$ 34
Changes in Loss and LAE Reserves				one-year