Loss And Loss Adjustment Expense Reserves (Schedule Of Company's Second-Lien RMBS Exposure, Gross Undiscounted Claim Liability And Potential Recoveries, Excluding Amounts Related To Consolidated VIE's) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Loss And Loss Adjustment Expense Reserves [Line Items]
Outstanding, Gross Principal	$ 15,187	[1]	$ 16,504	[1]
Outstanding, Gross Interest	8,526	[1]	8,984	[1]
Gross Undiscounted, Claim Liability	1,670		1,812
Gross Undiscounted, Potential Recoveries	3,844		3,813
Excluding Consolidated VIEs [Member] | Insured Issues Designated As "Classified List" [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Issues	22
Outstanding, Gross Principal	4,200
Outstanding, Gross Interest	1,700
Gross Undiscounted, Claim Liability	300
Gross Undiscounted, Potential Recoveries	3,200
Excluding Consolidated VIEs [Member] | Insured Issues Reviewed With Potential Recoveries [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Issues	15
Outstanding, Gross Principal	3,900
Outstanding, Gross Interest	1,600
Gross Undiscounted, Claim Liability	200
Gross Undiscounted, Potential Recoveries	3,100
Consolidated VIEs [Member] | Insured Issues Designated As "Classified List" [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Issues	12
Outstanding, Gross Principal	2,300
Outstanding, Gross Interest	900
Gross Undiscounted, Claim Liability	100
Gross Undiscounted, Potential Recoveries	1,400
Consolidated VIEs [Member] | Insured Issues Reviewed With Potential Recoveries [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Issues	11
Outstanding, Gross Principal	2,200
Outstanding, Gross Interest	800
Gross Undiscounted, Claim Liability	100
Gross Undiscounted, Potential Recoveries	$ 1,400

[1] Represents contractual principal and interest payments due by the issuer of the obligations insured by MBIA.