Loss And Loss Adjustment Expense Reserves (Schedule Of Financial Guarantees And Related Claim Liability) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 Policies Issues		Dec. 31, 2011 Issues Policies
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies	295		296
Number of issues	190	[1]	191	[1]
Gross insured contractual payments outstanding, Principal	$ 15,257	[2]	$ 16,504	[2]
Gross insured contractual payments outstanding, Interest	8,361	[2]	8,984	[2]
Gross insured contractual payments outstanding, Total	23,618	[2]	25,488	[2]
Gross claim liability	1,589		1,812
Less: Gross potential recoveries	4,109		3,813
Less: Discount, net	229		177
Net claim liability (recoverable)	(2,749)		(2,178)
Unearned premium revenue	278		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	54		54
Number of issues	29	[1]	32	[1]
Gross insured contractual payments outstanding, Principal	4,250	[2]	4,310	[2]
Gross insured contractual payments outstanding, Interest	2,721	[2]	2,653	[2]
Gross insured contractual payments outstanding, Total	6,971	[2]	6,963	[2]
Gross claim liability
Less: Gross potential recoveries
Less: Discount, net
Net claim liability (recoverable)
Unearned premium revenue	142		155
Remaining weighted average contract period (in years)	8 years 1 month 6 days		8 years 2 months 12 days
Caution List Medium [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies	25		28
Number of issues	15	[1]	18	[1]
Gross insured contractual payments outstanding, Principal	1,176	[2]	1,213	[2]
Gross insured contractual payments outstanding, Interest	256	[2]	351	[2]
Gross insured contractual payments outstanding, Total	1,432	[2]	1,564	[2]
Gross claim liability
Less: Gross potential recoveries
Less: Discount, net
Net claim liability (recoverable)
Unearned premium revenue	11		16
Remaining weighted average contract period (in years)	4 years		5 years 7 months 6 days
Caution List High [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies	10		14
Number of issues	10	[1]	11	[1]
Gross insured contractual payments outstanding, Principal	373	[2]	561	[2]
Gross insured contractual payments outstanding, Interest	120	[2]	144	[2]
Gross insured contractual payments outstanding, Total	493	[2]	705	[2]
Gross claim liability
Less: Gross potential recoveries
Less: Discount, net
Net claim liability (recoverable)
Unearned premium revenue	3		3
Remaining weighted average contract period (in years)	7 years 7 months 6 days		6 years
Classified List [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies	206		200
Number of issues	136	[1]	130	[1]
Gross insured contractual payments outstanding, Principal	9,458	[2]	10,420	[2]
Gross insured contractual payments outstanding, Interest	5,264	[2]	5,836	[2]
Gross insured contractual payments outstanding, Total	14,722	[2]	16,256	[2]
Gross claim liability	1,589		1,812
Less: Gross potential recoveries	4,109		3,813
Less: Discount, net	229		177
Net claim liability (recoverable)	(2,749)		(2,178)
Unearned premium revenue	$ 122		$ 134
Remaining weighted average contract period (in years)	9 years 6 months		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.