Variable Interest Entities (Summary Of Nonconsolidated VIEs Assets And Liabilities) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Variable Interest Entity [Line Items]
VIE Assets	$ 135.0
Collateralized Debt Obligations [Member]
Variable Interest Entity [Line Items]
VIE Assets	15,031.0		16,925.0
Maximum Exposure to Loss	10,796		10,873
Investments		[1]		[1]
Premiums Receivable	60	[2]	62	[2]
Insurance Loss Recoverable	4	[3]	5	[3]
Unearned Premium Revenue	53	[4]	55	[4]
Loss and Loss Adjustment Expense Reserves	35	[5]	37	[5]
Derivative Liabilities	66	[6]	74	[6]
Residential Mortgage-Backed Securities [Member]
Variable Interest Entity [Line Items]
VIE Assets	32,210.0		34,061.0
Maximum Exposure to Loss	12,505		13,075
Investments	10	[1]	11	[1]
Premiums Receivable	72	[2]	77	[2]
Insurance Loss Recoverable	3,502	[3]	3,278	[3]
Unearned Premium Revenue	70	[4]	75	[4]
Loss and Loss Adjustment Expense Reserves	389	[5]	440	[5]
Derivative Liabilities	5	[6]	4	[6]
Commercial Mortgage-Backed [Member]
Variable Interest Entity [Line Items]
VIE Assets	4,646.0		4,801.0
Maximum Exposure to Loss	2,328		2,432
Investments		[1]		[1]
Premiums Receivable	2	[2]	2	[2]
Insurance Loss Recoverable		[3]		[3]
Unearned Premium Revenue	2	[4]	2	[4]
Loss and Loss Adjustment Expense Reserves		[5]		[5]
Derivative Liabilities		[6]		[6]
Consumer Asset-Backed [Member]
Variable Interest Entity [Line Items]
VIE Assets	5,605.0		5,820.0
Maximum Exposure to Loss	2,990		3,086
Investments	10	[1]	10	[1]
Premiums Receivable	18	[2]	19	[2]
Insurance Loss Recoverable		[3]		[3]
Unearned Premium Revenue	18	[4]	19	[4]
Loss and Loss Adjustment Expense Reserves	21	[5]	21	[5]
Derivative Liabilities		[6]		[6]
Corporate Asset-Backed [Member]
Variable Interest Entity [Line Items]
VIE Assets	18,799.0		19,980.0
Maximum Exposure to Loss	9,309		9,981
Investments		[1]		[1]
Premiums Receivable	111	[2]	123	[2]
Insurance Loss Recoverable	13	[3]	13	[3]
Unearned Premium Revenue	128	[4]	140	[4]
Loss and Loss Adjustment Expense Reserves		[5]		[5]
Derivative Liabilities		[6]		[6]
Global Structured Finance [Member]
Variable Interest Entity [Line Items]
VIE Assets	76,291.0		81,587.0
Maximum Exposure to Loss	37,928		39,447
Investments	20	[1]	21	[1]
Premiums Receivable	263	[2]	283	[2]
Insurance Loss Recoverable	3,519	[3]	3,296	[3]
Unearned Premium Revenue	271	[4]	291	[4]
Loss and Loss Adjustment Expense Reserves	445	[5]	498	[5]
Derivative Liabilities	71	[6]	78	[6]
Global Public Finance [Member]
Variable Interest Entity [Line Items]
VIE Assets	45,298.0		39,259.0
Maximum Exposure to Loss	20,242		21,346
Investments		[1]		[1]
Premiums Receivable	202	[2]	220	[2]
Insurance Loss Recoverable		[3]		[3]
Unearned Premium Revenue	248	[4]	267	[4]
Loss and Loss Adjustment Expense Reserves	4	[5]	4	[5]
Derivative Liabilities		[6]		[6]
Total Insurance [Member]
Variable Interest Entity [Line Items]
VIE Assets	121,589.0		120,846.0
Maximum Exposure to Loss	58,170		60,793
Investments	20	[1]	21	[1]
Premiums Receivable	465	[2]	503	[2]
Insurance Loss Recoverable	3,519	[3]	3,296	[3]
Unearned Premium Revenue	519	[4]	558	[4]
Loss and Loss Adjustment Expense Reserves	449	[5]	502	[5]
Derivative Liabilities	$ 71	[6]	$ 78	[6]

[1] Reported within "Investments" on MBIA's consolidated balance sheets.

[2] Reported within "Premiums receivable" on MBIA's consolidated balance sheets.

[3] Reported within "Insurance loss recoverable" on MBIA's consolidated balance sheets.

[4] Reported within "Unearned premium revenue" on MBIA's consolidated balance sheets.

[5] Reported within "Loss and loss adjustment expense reserves" on MBIA's consolidated balance sheets.

[6] Reported within "Derivative liabilities" on MBIA's consolidated balance sheets.