Loss And Loss Adjustment Expense Reserves (Schedule OF Losses And Loss Adjustment Expenses Reserves and Recoveries) (Detail) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017
Loss And Lae Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loss and loss adjustment expense reserves		$ 1,033	$ 979
Insurance Loss Recoverable [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		1,542	511
U S Public Finance Insurance [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		554	333
Loss and loss adjustment expense reserves		634	512
International And Structured Finance Insurance [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable	[1]	1,457	1,478
Loss and loss adjustment expense reserves	[1]	628	710
International And Structured Finance Insurance [Member] | Variable Interest Entity Primary Beneficiary [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable	[1]	(469)	(1,300)
Loss and loss adjustment expense reserves	[1]	(229)	(243)
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		415	407
International And Structured Finance Insurance [Member] | Non Variable Interest Entity [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		988	178
Loss and loss adjustment expense reserves		$ 399	$ 467

[1] Includes loan repurchase commitments of $415 million and $407 million as of September 30, 2018 and December 31, 2017, respectively.