Loss And Loss Adjustment Expense Reserves (Schedule Of Losses And Loss Adjustment Expenses Reserves and Recoveries) (Detail) - USD ($) $ in Millions		Sep. 30, 2019		Dec. 31, 2018
Loss And Lae Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loss and loss adjustment expense reserves	[1],[2]	$ 859		$ 965
Insurance Loss Recoverable [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		1,843		1,595
U S Public Finance Insurance [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		945		571
Loss and loss adjustment expense reserves	[2]	438		551
U S Public Finance Insurance [Member] | Non Variable Interest Entity [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		945		571
Loss and loss adjustment expense reserves	[2]	374		551
International And Structured Finance Insurance [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable	[3]	1,389		1,461
Loss and loss adjustment expense reserves	[2],[3]	797		668
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		478		418
International And Structured Finance Insurance [Member] | Non Variable Interest Entity [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		898		1,024
Loss and loss adjustment expense reserves	[2]	485		414
Intersegment Elimination [Member] | U S Public Finance Insurance [Member] | Variable Interest Entity Primary Beneficiary [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		0		0
Loss and loss adjustment expense reserves		(64)	[2]	0
Intersegment 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]	(491)		(437)
Loss and loss adjustment expense reserves	[2],[3]	$ (312)		$ (254)

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

[2] Amounts are net of expected recoveries.

[3] Includes loan repurchase commitments of $478 million and $418 million as of September 30, 2019 and December 31, 2018, respectively.