Loss And Loss Adjustment Expense Reserves (Loss And LAE Activity) (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Loss And Loss Adjustment Expense Reserves [Line Items]
Weighted average risk-free rate used to discount claim liability			0.71%
Lae [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Losses and loss adjustment	$ 8	$ 16	$ 25	$ 26
Loss and loss adjustment expense reserves	$ 33		$ 33		$ 34
Changes in Loss and LAE Reserves			one-year