Loss And Loss Adjustment Expense Reserves (Schedule Of Losses And Loss Adjustment Expenses Reserves and Recoveries) (Detail) - USD ($) $ in Millions		Sep. 30, 2020	Dec. 31, 2019
Loss And Lae Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loss and loss adjustment expense reserves	[1],[2]	$ 991	$ 901
Insurance Loss Recoverable [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		1,798	1,694
Variable Interest Entity Primary Beneficiary [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loan repurchase commitments		530	486
Non Variable Interest Entity [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		1,798	1,694
Loss and loss adjustment expense reserves		991	901
U S Public Finance Insurance [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		1,233	911
Loss and loss adjustment expense reserves		454	432
International And Structured Finance Insurance [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable	[3]	1,116	1,286
Loss and loss adjustment expense reserves	[1],[3]	791	749
International And Structured Finance Insurance [Member] | Variable Interest Entity Primary Beneficiary [Member] | Loan Repurchase Commitments [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loan repurchase commitments		530	486
International And Structured Finance Insurance [Member] | Non Variable Interest Entity [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		565	783
Loss and loss adjustment expense reserves	[1]	537	469
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]	(551)	(503)
Loss and loss adjustment expense reserves	[1],[3]	$ (254)	$ (280)

[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 $530 million and $486 million as of September 30, 2020 and December 31, 2019, respectively.