Loss And Loss Adjustment Expense Reserves (Schedule Of Losses And Loss Adjustment Expenses Reserves and Recoveries) (Detail) - USD ($) $ in Millions		Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Loss And Lae Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loss and loss adjustment expense reserves	[2]	$ 990	[1]	$ 901	[1]	$ 965
Insurance Loss Recoverable [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		1,677		1,694		$ 1,595
Variable Interest Entity Primary Beneficiary [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loan repurchase commitments		604		486
Non Variable Interest Entity [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		1,677		1,694
Loss and loss adjustment expense reserves		990		901
U S Public Finance Insurance [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		1,220		911
Loss and loss adjustment expense reserves	[1]	469		432
International And Structured Finance Insurance [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable	[3]	1,082		1,286
Loss and loss adjustment expense reserves	[1],[3]	780		749
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		604		486
International And Structured Finance Insurance [Member] | Non Variable Interest Entity [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance Loss Recoverable		457		783
Loss and loss adjustment expense reserves	[1]	521		469
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]	(625)		(503)
Loss and loss adjustment expense reserves	[1],[3]	$ (259)		$ (280)

[1] Amounts are net of expected recoveries.

[2] Includes changes in amount and timing of estimated payments and recoveries

[3] Includes loan repurchase commitments of $604 million and $486 million as of December 31, 2020 and 2019, respectively.