Loss And Loss Adjustment Expense Reserves (Schedule Of Losses And Loss Adjustment Expenses Reserves and Recoveries) (Detail) - USD ($) $ in Millions		Jun. 30, 2021	Dec. 31, 2020
Loss And Lae Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loss and loss adjustment expense reserves	[1],[2]	$ 954	$ 990
Insurance Loss Recoverable [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		1,561	1,677
Non Variable Interest Entity [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		1,561	1,677
Loss and loss adjustment expense reserves		954	990
US Public Finance [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		1,226	1,220
Loss and loss adjustment expense reserves	[1]	484	469
International And Structured Finance Insurance [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable	[3]	354	1,082
Loss and loss adjustment expense reserves	[1],[3]	697	780
International And Structured Finance Insurance [Member] | Non Variable Interest Entity [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		335	457
Loss and loss adjustment expense reserves	[1]	470	521
Consolidation Elimination [Member] | International And Structured Finance Insurance [Member] | Variable Interest Entity Primary Beneficiary [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable	[3]	(19)	(625)
Loss and loss adjustment expense reserves	[1],[3]	$ (227)	$ (259)

[1] Amounts are net of expected recoveries.

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

[3] Includes loan repurchase commitments of $604 million as of December 31, 2020.