Loss And Loss Adjustment Expense Reserves (RMBS Reserves) (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014	Dec. 31, 2014
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance loss recoverable	$ 527			$ 533
Schedule Of Insured Financial Obligations With Credit Deterioration Insured Contractual Payments Outstanding Principal	10,884	[1]		11,263	[1]
Policyholder (Benefits) And Claims Incurred Net	(6)		50
Contract claims on ineligible mortgage loans inception to date, incurred loss on transactions	418
Loss And Lae Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loss and loss adjustment expense reserves	499			506
Expected recoveries of future payments	110			147
First Lien R M B S Reserves [Member] | Loss And Lae Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loss and loss adjustment expense reserves	267			263
Second Lien Rmbs And Heloc And Ces [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Addition to case basis reserves if losses remain at peak levels	40
Second Lien Rmbs And Heloc And Ces [Member] | Excess Spread [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance loss recoverable	488			496
Total excess spread recoveries	502			523
Expected recoveries of future payments	14			27
Second Lien Rmbs And Heloc And Ces [Member] | Excess Spread [Member] | Variable Interest Entity Primary Beneficiary [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance loss recoverable	122			132
Total excess spread recoveries	124			137
Expected recoveries of future payments	2			5
U S Public Finance Insurance [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loss and loss adjustment expense reserves	35			45
Insurance loss recoverable	4			4
Policyholder (Benefits) And Claims Incurred Net	(6)		(14)
U S Public Finance Insurance [Member] | General obligations
Loss And Loss Adjustment Expense Reserves [Line Items]
Schedule Of Insured Financial Obligations With Credit Deterioration Insured Contractual Payments Outstanding Principal	92,500			98,900
U S Public Finance Insurance [Member] | General obligations | Classified [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Schedule Of Insured Financial Obligations With Credit Deterioration Insured Contractual Payments Outstanding Principal	83			152
Second Lien R M B S Securitizations [Member] | Loss And Lae Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loss and loss adjustment expense reserves	71			70
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	$ 29			$ 21

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