Loss And Loss Adjustment Expense Reserves (Schedule Of Loss And Loss Adjustment Expenses Reserves) (Detail) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019
Loss And Loss Adjustment Expense Reserves [Line Items]
Changes in unearned premium revenue		$ 53	$ 88
Loss And Lae Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Gross loss and LAE reserve, beginning balance	[1],[2]	901
Loss payments for cases		(435)
Accretion of claim liability discount		8
Changes in discount rates		(86)
Changes in assumptions		600
Changes in unearned premium revenue		7
Other		(4)
Gross loss and LAE reserve, ending balance	[1],[2]	$ 991

[1] Amounts are net of expected recoveries.

[2] Includes changes in amount and timing of estimated payments and recoveries.