Loss And Loss Adjustment Expense Reserves (RMBS Reserves) (Narrative) (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance loss recoverable		$ 483		$ 483		$ 533
Schedule Of Insured Financial Obligations With Credit Deterioration Insured Contractual Payments Outstanding Principal	[1]	12,212		12,212		11,263
Policyholder (Benefits) And Claims Incurred Net		46	$ 12	40	$ 62
Contract claims on ineligible mortgage loans inception to date, incurred loss on transactions		422		422
Loss And Lae Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loss and loss adjustment expense reserves		500		500		506
Expected recoveries of future payments		219		219		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		235		235		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		29
Second Lien Rmbs And Heloc And Ces [Member] | Excess Spread [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance loss recoverable		445		445		496
Total excess spread recoveries		451		451		523
Expected recoveries of future payments		6		6		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		108		108		132
Total excess spread recoveries		108		108		137
Expected recoveries of future payments						5
U S Public Finance Insurance [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loss and loss adjustment expense reserves		42		42		45
Insurance loss recoverable		1		1		4
Policyholder (Benefits) And Claims Incurred Net		8	$ 17	2	$ 3
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		84,700		84,700		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		82		82		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		72		72		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		$ 24		$ 24		$ 21

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