Loss And Loss Adjustment Expense Reserves (Schedule OF Losses And Loss Adjustment Expenses Reserves and Recoveries) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Loss And Loss Adjustment Expense Reserves [Line Items]
Loan repurchase commitments	$ 407		$ 407
Insurance Loss Recoverable	530		511
Loss and loss adjustment expense reserves	1,006		979
Losses and loss adjustment	72	$ 94
Loss And Lae Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loss and loss adjustment expense reserves	1,006		979
Insurance Loss Recoverable [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable	530		511
Second Lien Residential Mortgage Backed Securities And Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Losses and loss adjustment	2	23
CDOs [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Losses and loss adjustment	5	$ 2
U S Public Finance Insurance [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable	364		333
Loss and loss adjustment expense reserves	553		512
International And Structured Finance Insurance [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loan repurchase commitments	407		407
Insurance Loss Recoverable	166		178
Loss and loss adjustment expense reserves	453		467
International And Structured Finance Insurance [Member] | Variable Interest Entity Primary Beneficiary [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable	(1,300)		(1,300)
Loss and loss adjustment expense reserves	(241)		(243)
International And Structured Finance Insurance [Member] | Non Variable Interest Entity [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loan repurchase commitments	407		407
Insurance Loss Recoverable	1,466		1,478
Loss and loss adjustment expense reserves	$ 694		$ 710