Loss And Loss Adjustment Expense Reserves (Schedule Of Financial Guarantees And Related Claim Liability) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2012 Policies Issues		Dec. 31, 2011 Issues Policies
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies	293		296
Number of issues	188	[1]	191	[1]
Gross insured contractual payments outstanding, Principal	$ 15,187	[2]	$ 16,504	[2]
Gross insured contractual payments outstanding, Interest	8,526	[2]	8,984	[2]
Gross insured contractual payments outstanding, Total	23,713	[2]	25,488	[2]
Gross claim liability	1,670		1,812
Less: Gross potential recoveries	3,844		3,813
Less: Discount, net	178		177
Net claim liability (recoverable)	(2,352)		(2,178)
Unearned premium revenue	269		308
Remaining weighted average contract period (in years)	8 years 8 months 12 days		8 years 9 months 18 days
Caution List Low [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies	49		54
Number of issues	27	[1]	32	[1]
Gross insured contractual payments outstanding, Principal	3,747	[2]	4,310	[2]
Gross insured contractual payments outstanding, Interest	2,690	[2]	2,653	[2]
Gross insured contractual payments outstanding, Total	6,437	[2]	6,963	[2]
Unearned premium revenue	130		155
Remaining weighted average contract period (in years)	8 years 8 months 12 days		8 years 2 months 12 days
Caution List Medium [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies	24		28
Number of issues	14	[1]	18	[1]
Gross insured contractual payments outstanding, Principal	1,409	[2]	1,213	[2]
Gross insured contractual payments outstanding, Interest	334	[2]	351	[2]
Gross insured contractual payments outstanding, Total	1,743	[2]	1,564	[2]
Unearned premium revenue	17		16
Remaining weighted average contract period (in years)	4 years 7 months 6 days		5 years 7 months 6 days
Caution List High [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies	11		14
Number of issues	10	[1]	11	[1]
Gross insured contractual payments outstanding, Principal	360	[2]	561	[2]
Gross insured contractual payments outstanding, Interest	115	[2]	144	[2]
Gross insured contractual payments outstanding, Total	475	[2]	705	[2]
Unearned premium revenue	3		3
Remaining weighted average contract period (in years)	7 years 9 months 18 days		6 years
Classified List [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies	209		200
Number of issues	137	[1]	130	[1]
Gross insured contractual payments outstanding, Principal	9,671	[2]	10,420	[2]
Gross insured contractual payments outstanding, Interest	5,387	[2]	5,836	[2]
Gross insured contractual payments outstanding, Total	15,058	[2]	16,256	[2]
Gross claim liability	1,670		1,812
Less: Gross potential recoveries	3,844		3,813
Less: Discount, net	178		177
Net claim liability (recoverable)	(2,352)		(2,178)
Unearned premium revenue	$ 119		$ 134
Remaining weighted average contract period (in years)	9 years 4 months 24 days		9 years 7 months 6 days

[1] An "issue" represents the aggregate of financial guarantee policies that share the same revenue source for purposes of making debt service payments.

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