Loss And Loss Adjustment Expense Reserves (RMBS Reserves) (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended			9 Months Ended					9 Months Ended					0 Months Ended	6 Months Ended	9 Months Ended	0 Months Ended	6 Months Ended		24 Months Ended	6 Months Ended		9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012	Sep. 30, 2013		Sep. 30, 2012	Dec. 31, 2012		Sep. 30, 2013 First Lien R M B S Reserves [Member]	Sep. 30, 2013 First Lien R M B S Reserves [Member] ALT-A [Member]	Sep. 30, 2013 First Lien R M B S Reserves [Member] Foreclosure [Member]	Sep. 30, 2013 First Lien R M B S Reserves [Member] Real Estate Owned [Member]	Sep. 30, 2013 First Lien R M B S Reserves [Member] Bankruptcy [Member]	Aug. 31, 2013 Rmbs And Heloc And Ces [Member]	Feb. 28, 2014 Rmbs And Heloc And Ces [Member]	Sep. 30, 2013 Rmbs And Heloc And Ces [Member]	Jun. 30, 2014 Rmbs And Heloc And Ces [Member] Scenario Forecast [Member]	Feb. 28, 2014 Rmbs And Heloc And Ces [Member] Scenario Forecast [Member]	Feb. 28, 2014 Rmbs And Heloc And Ces [Member] Scenario Forecast [Member] Minimum [Member]	Aug. 31, 2015 Rmbs And Heloc And Ces [Member] Scenario Forecast [Member] Maximum [Member]	Feb. 28, 2014 Rmbs And Heloc And Ces [Member] Scenario Forecast [Member] Rate Reduction [Member]	Sep. 30, 2013 Rmbs And Heloc And Ces [Member] Excess Spread [Member]	Sep. 30, 2013 U S Public Finance Insurance [Member]	Sep. 30, 2013 U S Public Finance Insurance [Member] General obligations	Sep. 30, 2013 U S Public Finance Insurance [Member] General obligations Classified [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loss and loss adjustment expense reserves																							$ 94
Insurance loss recoverable																						669	17
Schedule Of Insured Financial Obligations With Credit Deterioration Insured Contractual Payments Outstanding Principal	15,182	[1]		15,182	[1]		15,257	[1]																131,500	251
Policyholder Benefits And Claims Incurred Net	98		171	92		330																	105
Charged-off loans recovery values				0
Delinquency period charged off on transaction specific basis, days									90
Total excess spread recoveries																						756
Expected recoveries of future payments																						87
Assumed roll rate for loans											90.00%	90.00%	75.00%
90+ day roll rate to loss set stress scenario									90.00%
Percentage of loss after declining over a 24-month period										25.00%
Assumed roll rate for ninety plus day delinquent loans excluding foreclosure and real estate owned																95.00%
Period of elevated delinquency and loss for base case, beginning, months															6
Percentage of loans in 30-59 days delinquent bucket														10.00%
Percentage of loans charged-off														30.00%
Percentage of Current Roll to Loss for the transaction														3.00%
Decrease in percentage of Current Roll to Loss for transaction																		25.00%			0.75%
Current Roll to Loss																	0.00%
Period of elevated delinquency and loss for stress case, additional months																			6	24
Addition to case basis reserves before considering potential recoveries for elevated delinquency and loss for stress case																$ 85

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