Loss And Loss Adjustment Expense Reserves (Schedule OF Losses And Loss Adjustment Expenses Reserves and Recoveries) (Detail) - USD ($) $ in Millions		Jun. 30, 2018	Dec. 31, 2017
Loss And Lae Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loss and loss adjustment expense reserves		$ 1,035	$ 979
Insurance Loss Recoverable [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		1,359	511
U S Public Finance Insurance [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		365	333
Loss and loss adjustment expense reserves		610	512
International And Structured Finance Insurance [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable	[1]	1,455	1,478
Loss and loss adjustment expense reserves	[1]	657	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]	(461)	(1,300)
Loss and loss adjustment expense reserves	[1]	(232)	(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		994	178
Loss and loss adjustment expense reserves		$ 425	$ 467

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