Variable Interest Entities (Summary Of Nonconsolidated VIEs Assets And Liabilities) (Detail) - USD ($) $ in Millions		Jun. 30, 2018		Dec. 31, 2017
Global Structured Finance [Member]
Variable Interest Entity [Line Items]
VIE Assets		$ 15,022		$ 14,939
Maximum Exposure to Loss		5,708		6,461
Investments	[1]	19		19
Premiums receivable	[2]	33		39
Insurance loss recoverable	[3]	989		173
Unearned premium revenue	[4]	33		37
Loss and loss adjustment expense reserves	[5]	371		406
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member]
Variable Interest Entity [Line Items]
VIE Assets		6,187		7,295
Maximum Exposure to Loss		3,396		3,741
Investments	[1]	19		19
Premiums receivable	[2]	20		22
Insurance loss recoverable	[3]	149		172
Unearned premium revenue	[4]	19		20
Loss and loss adjustment expense reserves	[5]	361		396
Global Structured Finance [Member] | Commercial Mortgage Backed Securities [Member]
Variable Interest Entity [Line Items]
VIE Assets		139		216
Maximum Exposure to Loss		67		94
Premiums receivable		0	[2]	0
Unearned premium revenue		0	[4]	0
Global Structured Finance [Member] | Consumer Asset Backed [Member]
Variable Interest Entity [Line Items]
VIE Assets		4,787		5,010
Maximum Exposure to Loss		798		981
Premiums receivable	[2]	3		4
Insurance loss recoverable	[3]	1		1
Unearned premium revenue	[4]	3		3
Loss and loss adjustment expense reserves	[5]	10		10
Global Structured Finance [Member] | Corporate Asset Backed [Member]
Variable Interest Entity [Line Items]
VIE Assets		3,909		2,418
Maximum Exposure to Loss		1,447		1,645
Investments	[1]	0		0
Premiums receivable	[2]	10		13
Insurance loss recoverable		839	[3]	0
Unearned premium revenue	[4]	11		14
Global Public Finance [Member]
Variable Interest Entity [Line Items]
VIE Assets		15,317		15,568
Maximum Exposure to Loss		2,352		2,524
Premiums receivable	[2]	10		10
Unearned premium revenue	[4]	12		14
Total Insurance [Member]
Variable Interest Entity [Line Items]
VIE Assets		30,339		30,507
Maximum Exposure to Loss		8,060		8,985
Investments	[1]	19		19
Premiums receivable	[2]	43		49
Insurance loss recoverable	[3]	989		173
Unearned premium revenue	[4]	45		51
Loss and loss adjustment expense reserves	[5]	$ 371		$ 406

[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.