Loss And Loss Adjustment Expense Reserves (Schedule Of Financial Guarantees And Related Claim Liability) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013 Policies Issues		Dec. 31, 2012 Issues Policies
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies	295		295
Number of issues	193	[1]	190	[1]
Gross insured contractual payments outstanding, Principal	$ 14,837	[2]	$ 15,257	[2]
Gross insured contractual payments outstanding, Interest	8,294	[2]	8,361	[2]
Gross insured contractual payments outstanding, Total	23,131	[2]	23,618	[2]
Gross claim liability	1,459		1,589
Less: Gross potential recoveries	4,276		4,109
Less: Discount, net	234		229
Net claim liability (recoverable)	(3,051)		(2,749)
Unearned premium revenue	265		278
Remaining weighted average contract period (in years)	8 years 9 months 18 days		8 years 8 months 12 days
Caution List Low [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies	54		54
Number of issues	31	[1]	29	[1]
Gross insured contractual payments outstanding, Principal	3,893	[2]	4,250	[2]
Gross insured contractual payments outstanding, Interest	2,526	[2]	2,721	[2]
Gross insured contractual payments outstanding, Total	6,419	[2]	6,971	[2]
Gross claim liability
Less: Gross potential recoveries
Less: Discount, net
Net claim liability (recoverable)
Unearned premium revenue	125		142
Remaining weighted average contract period (in years)	7 years 10 months 24 days		8 years 1 month 6 days
Caution List Medium [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies	24		25
Number of issues	15	[1]	15	[1]
Gross insured contractual payments outstanding, Principal	1,466	[2]	1,176	[2]
Gross insured contractual payments outstanding, Interest	359	[2]	256	[2]
Gross insured contractual payments outstanding, Total	1,825	[2]	1,432	[2]
Gross claim liability
Less: Gross potential recoveries
Less: Discount, net
Net claim liability (recoverable)
Unearned premium revenue	22		11
Remaining weighted average contract period (in years)	4 years 10 months 24 days		4 years
Caution List High [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies	8		10
Number of issues	8	[1]	10	[1]
Gross insured contractual payments outstanding, Principal	232	[2]	373	[2]
Gross insured contractual payments outstanding, Interest	97	[2]	120	[2]
Gross insured contractual payments outstanding, Total	329	[2]	493	[2]
Gross claim liability
Less: Gross potential recoveries
Less: Discount, net
Net claim liability (recoverable)
Unearned premium revenue	2		3
Remaining weighted average contract period (in years)	8 years 1 month 6 days		7 years 7 months 6 days
Classified List [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies	209		206
Number of issues	139	[1]	136	[1]
Gross insured contractual payments outstanding, Principal	9,246	[2]	9,458	[2]
Gross insured contractual payments outstanding, Interest	5,312	[2]	5,264	[2]
Gross insured contractual payments outstanding, Total	14,558	[2]	14,722	[2]
Gross claim liability	1,459		1,589
Less: Gross potential recoveries	4,276		4,109
Less: Discount, net	234		229
Net claim liability (recoverable)	(3,051)		(2,749)
Unearned premium revenue	$ 116		$ 122
Remaining weighted average contract period (in years)	9 years 9 months 18 days		9 years 6 months

[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.