Variable Interest Entities (Summary Of Nonconsolidated VIEs Assets And Liabilities) (Detail) - USD ($) $ in Millions		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Variable Interest Entity [Line Items]
Premiums receivable		$ 369		$ 409	$ 792
Insurance loss recoverable		511		504	$ 577	[1]
Loss and loss adjustment expense reserves		979		541
Global Structured Finance [Member]
Variable Interest Entity [Line Items]
VIE Assets		14,939		20,088
Maximum Exposure to Loss		6,461		10,391
Investments	[2]	19		76
Premiums receivable	[3]	39		55
Insurance loss recoverable	[4]	173		306
Unearned premium revenue	[5]	37		52
Loss and loss adjustment expense reserves	[6]	406		406
Global Structured Finance [Member] | Collateralized Debt Obligations [Member]
Variable Interest Entity [Line Items]
VIE Assets		0		3,167
Maximum Exposure to Loss		0		1,914
Investments	[2]	0		51
Premiums receivable	[3]	0		2
Unearned premium revenue		0	[5]	0
Loss and loss adjustment expense reserves	[6]	0		73
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member]
Variable Interest Entity [Line Items]
VIE Assets		7,295		9,146
Maximum Exposure to Loss		3,741		4,796
Investments	[2]	19		20
Premiums receivable	[3]	22		28
Insurance loss recoverable	[4]	172		304
Unearned premium revenue	[5]	20		27
Loss and loss adjustment expense reserves	[6]	396		325
Global Structured Finance [Member] | Commercial Mortgage Backed Securities [Member]
Variable Interest Entity [Line Items]
VIE Assets		216		257
Maximum Exposure to Loss		94		145
Premiums receivable		0	[3]	0
Unearned premium revenue		0	[5]	0
Global Structured Finance [Member] | Consumer Asset Backed [Member]
Variable Interest Entity [Line Items]
VIE Assets		5,010		4,893
Maximum Exposure to Loss		981		1,331
Premiums receivable	[3]	4		7
Insurance loss recoverable	[4]	1		2
Unearned premium revenue	[5]	3		5
Loss and loss adjustment expense reserves	[6]	10		8
Global Structured Finance [Member] | Corporate Asset Backed [Member]
Variable Interest Entity [Line Items]
VIE Assets		2,418		2,625
Maximum Exposure to Loss		1,645		2,205
Investments	[2]	0		5
Premiums receivable	[3]	13		18
Insurance loss recoverable		0	[4]	0
Unearned premium revenue	[5]	14		20
Global Public Finance [Member]
Variable Interest Entity [Line Items]
VIE Assets		15,568		44,306
Maximum Exposure to Loss		2,524		12,051
Premiums receivable	[3]	10		11
Unearned premium revenue	[5]	14		18
Total Insurance [Member]
Variable Interest Entity [Line Items]
VIE Assets		30,507		64,394
Maximum Exposure to Loss		8,985		22,442
Investments	[2]	19		76
Premiums receivable	[3]	49		66
Insurance loss recoverable	[4]	173		306
Unearned premium revenue	[5]	51		70
Loss and loss adjustment expense reserves	[6]	$ 406		$ 406

[1] As of December 31, 2017 and December 31, 2016, excludes Puerto Rico recoveries, and as of December 31, 2016, the Zohar II recoveries, which have been netted against reserves.

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

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

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

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

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