Loss And Loss Adjustment Expense Reserves (Schedule Of Loss And Loss Adjustment Expenses Reserves) (Detail) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020
Loss And Loss Adjustment Expense Reserves [Line Items]
Changes in unearned premium revenue		$ 72	$ 53
Loss And Lae Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Gross loss and LAE reserve, beginning balance	[1],[2]	990
Loss payments for cases	[2]	(298)
Accretion of claim liability discount	[2]	11
Changes in discount rates	[2]	(25)
Changes in assumptions	[2]	173
Changes in unearned premium revenue	[2]	3
Other	[2]	(4)
Gross loss and LAE reserve, ending balance	[1],[2]	$ 850

[1] Amounts are net of expected recoveries.

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