Loss And Loss Adjustment Expense Reserves (Schedule Of Loss And Loss Adjustment Expenses Reserves) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Loss And Loss Adjustment Expense Reserves [Line Items]
Changes in unearned premium revenue	$ 105		$ 165		$ 206
Loss And Lae Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Gross loss and LAE reserve, beginning balance	965	[1],[2]	1,013
Loss payments for cases	431		354
Accretion of claim liability discount	18		26
Changes in discount rates	54		13
Changes in assumptions	407		258
Changes in unearned premium revenue	23		15
Changes in LAE	26		6
Other	1
Gross loss and LAE reserve, ending balance	$ 901	[1],[2]	$ 965	[1],[2]	$ 1,013

[1] Amounts are net of expected recoveries of unpaid claims.

[2] Amounts are net of expected recoveries.