Loss And Loss Adjustment Expense Reserves (RMBS Reserves) (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended									3 Months Ended				3 Months Ended				0 Months Ended	3 Months Ended	6 Months Ended	0 Months Ended	6 Months Ended		24 Months Ended	6 Months Ended			3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013	Dec. 31, 2013		Mar. 31, 2014 Loss And Lae Reserves [Member]	Dec. 31, 2013 Loss And Lae Reserves [Member]	Mar. 31, 2014 Classified [Member] Loss And Lae Reserves [Member]	Dec. 31, 2013 Classified [Member] Loss And Lae Reserves [Member]	Mar. 31, 2014 First Lien R M B S Reserves [Member]	Mar. 31, 2014 First Lien R M B S Reserves [Member] Loss And Lae Reserves [Member]	Dec. 31, 2013 First Lien R M B S Reserves [Member] Loss And Lae Reserves [Member]	Dec. 31, 2013 First Lien R M B S Reserves [Member] Variable Interest Entity Primary Beneficiary [Member]	Mar. 31, 2014 First Lien R M B S Reserves [Member] ALT-A [Member]	Mar. 31, 2014 First Lien R M B S Reserves [Member] Foreclosure [Member]	Mar. 31, 2014 First Lien R M B S Reserves [Member] Real Estate Owned [Member]	Mar. 31, 2014 First Lien R M B S Reserves [Member] Bankruptcy [Member]	Feb. 28, 2014 Rmbs And Heloc And Ces [Member]	Mar. 31, 2014 Rmbs And Heloc And Ces [Member]	Aug. 31, 2014 Rmbs And Heloc And Ces [Member]	Sep. 30, 2014 Rmbs And Heloc And Ces [Member] Scenario Forecast [Member]	Aug. 31, 2014 Rmbs And Heloc And Ces [Member] Scenario Forecast [Member]	Aug. 31, 2014 Rmbs And Heloc And Ces [Member] Scenario Forecast [Member] Minimum [Member]	Feb. 29, 2016 Rmbs And Heloc And Ces [Member] Scenario Forecast [Member] Maximum [Member]	Aug. 31, 2014 Rmbs And Heloc And Ces [Member] Scenario Forecast [Member] Rate Reduction [Member]	Mar. 31, 2014 Rmbs And Heloc And Ces [Member] Excess Spread [Member]	Dec. 31, 2013 Rmbs And Heloc And Ces [Member] Excess Spread [Member]	Mar. 31, 2014 U S Public Finance Insurance [Member]	Dec. 31, 2013 U S Public Finance Insurance [Member]	Mar. 31, 2014 U S Public Finance Insurance [Member] General obligations	Dec. 31, 2013 U S Public Finance Insurance [Member] General obligations	Mar. 31, 2014 U S Public Finance Insurance [Member] General obligations Classified [Member]	Dec. 31, 2013 U S Public Finance Insurance [Member] General obligations Classified [Member]	Mar. 31, 2014 Second Lien R M B S Securitizations [Member] Loss And Lae Reserves [Member]	Dec. 31, 2013 Second Lien R M B S Securitizations [Member] Loss And Lae Reserves [Member]	Mar. 31, 2014 Second Lien R M B S Securitizations [Member] Variable Interest Entity Primary Beneficiary [Member]	Dec. 31, 2013 Second Lien R M B S Securitizations [Member] Variable Interest Entity Primary Beneficiary [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loss and loss adjustment expense reserves						$ 602	$ 641	$ 602	$ 641		$ 246	$ 241	$ 2															$ 62	$ 87					$ 109	$ 126	$ 38	$ 43
Insurance loss recoverable																										626	647	9	13
Schedule Of Insured Financial Obligations With Credit Deterioration Insured Contractual Payments Outstanding Principal	16,032	[1]		14,264	[1]																									120,100	124,900	161	161
Policyholder (Benefits) And Claims Incurred Net	50		(194)																									(14)
Charged-off loans recovery values	0
Total excess spread recoveries																										651	681
Expected recoveries of future payments						182	206																			25	34
Assumed roll rate for loans															90.00%	90.00%	75.00%
90+ day roll rate to loss set stress scenario										90.00%
Percentage of roll to 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 if losses remain at peak levels																			$ 70

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