Loss And Loss Adjustment Expense Reserves (Schedule OF Losses And Loss Adjustment Expenses Reserves and Recoveries) (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Loss And Loss Adjustment Expense Reserves [Line Items]
Loan repurchase commitments	$ 406	$ 404
Insurance Loss Recoverable	611	504
Loss and loss adjustment expense reserves	818	541
Loss And Lae Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loss and loss adjustment expense reserves	818	541
Insurance Loss Recoverable [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable	611	504
U S Public Finance Insurance [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable	356	174
Loss and loss adjustment expense reserves	348	97
International And Structured Finance Insurance [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loan repurchase commitments	406	404
Insurance Loss Recoverable	255	330
Loss and loss adjustment expense reserves	470	444
International And Structured Finance Insurance [Member] | Variable Interest Entity Primary Beneficiary [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable	(1,304)	(221)
Loss and loss adjustment expense reserves	(224)	(206)
International And Structured Finance Insurance [Member] | Non Variable Interest Entity [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loan repurchase commitments	406	404
Insurance Loss Recoverable	1,559	551
Loss and loss adjustment expense reserves	$ 694	$ 650