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	Mar. 31, 2013		Dec. 31, 2012
Loss And Loss Adjustment Expense Reserves [Line Items]
Outstanding, Gross Principal	$ 14,837	[1]	$ 15,257	[1]
Outstanding, Gross Interest	8,294	[1]	8,361	[1]
Gross Undiscounted, Claim Liability	1,459		1,589
Gross Undiscounted, Potential Recoveries	4,276		4,109
Excluding Consolidated VIEs [Member] | Insured Issues Designated As "Classified List" [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Issues	23
Outstanding, Gross Principal	4,000
Outstanding, Gross Interest	1,500
Gross Undiscounted, Claim Liability	200
Gross Undiscounted, Potential Recoveries	3,700
Excluding Consolidated VIEs [Member] | Insured Issues Reviewed With Potential Recoveries [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Issues	16
Outstanding, Gross Principal	3,600
Outstanding, Gross Interest	1,400
Gross Undiscounted, Claim Liability	200
Gross Undiscounted, Potential Recoveries	3,600
Consolidated VIEs [Member] | Insured Issues Designated As "Classified List" [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Issues	12
Outstanding, Gross Principal	2,100
Outstanding, Gross Interest	800
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	2,000
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.