Loss And Loss Adjustment Expense Reserves (Schedule OF Losses And Loss Adjustment Expenses Reserves and Recoveries) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loss And Loss Adjustment Expense Reserves [Line Items]
Loan repurchase commitments	$ 407	$ 404
Insurance Loss Recoverable	511	504	$ 577	[1]
Loss and loss adjustment expense reserves	979	541
Losses and loss adjustment	683	220	123
Loss And Lae Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loss and loss adjustment expense reserves	979	541	516
Insurance Loss Recoverable [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable	511	504	577
Second Lien Residential Mortgage Backed Securities And Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Losses and loss adjustment	62	121	63
CDOs [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Losses and loss adjustment	10	(32)	$ 29
U S Public Finance Insurance [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable	333	174
Loss and loss adjustment expense reserves	512	97
International And Structured Finance Insurance [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loan repurchase commitments	407	404
Insurance Loss Recoverable	178	330
Loss and loss adjustment expense reserves	467	444
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)	(221)
Loss and loss adjustment expense reserves	(243)	(206)
International And Structured Finance Insurance [Member] | Non Variable Interest Entity [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loan repurchase commitments	407	404
Insurance Loss Recoverable	1,478	551
Loss and loss adjustment expense reserves	$ 710	$ 650

[1] As of December 31, 2017 and December 31, 2016, excludes Puerto Rico recoveries, and as of December 31, 2016, the Zohar II recoveries, which have been netted against reserves.