Loss And Loss Adjustment Expense Reserves (Schedule Of Losses And Loss Adjustment Expenses Reserves and Recoveries) (Detail) - USD ($) $ in Millions		Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Loss And Lae Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loss and loss adjustment expense reserves		$ 901	[1],[2]	$ 965	[1],[2]	$ 1,013
Insurance Loss Recoverable [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		1,694		1,595		$ 545
Variable Interest Entity Primary Beneficiary [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loan repurchase commitments		486		418
Non Variable Interest Entity [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		1,694		1,595
Loss and loss adjustment expense reserves		901		965
U S Public Finance Insurance [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		911		571
Loss and loss adjustment expense reserves	[2]	432		551
International And Structured Finance Insurance [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable	[3]	1,286		1,461
Loss and loss adjustment expense reserves	[2],[3]	749		668
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		486		418
International And Structured Finance Insurance [Member] | Non Variable Interest Entity [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		783		1,024
Loss and loss adjustment expense reserves	[2]	469		414
Consolidation Elimination [Member] | International And Structured Finance Insurance [Member] | Variable Interest Entity Primary Beneficiary [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable	[3]	(503)		(437)
Loss and loss adjustment expense reserves	[2],[3]	$ (280)		$ (254)

[1] Amounts are net of expected recoveries of unpaid claims.

[2] Amounts are net of expected recoveries.

[3] Includes loan repurchase commitments of $486 million and $418 million as of December 31, 2019 and 2018, respectively.