Variable Interest Entities (Summary Of Nonconsolidated VIEs Assets And Liabilities) (Detail) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Maximum Exposure to Loss		$ 7,284	$ 8,985
Investments [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	[1]	17	19
Premiums Receivable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	[2]	40	49
Insurance Loss Recoverable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	[3]	987	173
Unearned Premium Revenue [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	[4]	41	51
Loss And Loss Adjustment Expense Reserves Member [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	[5]	357	406
Global Structured Finance [Member]
Variable Interest Entity [Line Items]
Maximum Exposure to Loss		5,053	6,461
Global Structured Finance [Member] | Investments [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	[1]	17	19
Global Structured Finance [Member] | Premiums Receivable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	[2]	31	39
Global Structured Finance [Member] | Insurance Loss Recoverable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	[3]	987	173
Global Structured Finance [Member] | Unearned Premium Revenue [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	[4]	29	37
Global Structured Finance [Member] | Loss And Loss Adjustment Expense Reserves Member [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	[5]	357	406
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member]
Variable Interest Entity [Line Items]
Maximum Exposure to Loss		3,103	3,741
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member] | Investments [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	[1]	17	19
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member] | Premiums Receivable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	[2]	19	22
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member] | Insurance Loss Recoverable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	[3]	128	172
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member] | Unearned Premium Revenue [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	[4]	17	20
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member] | Loss And Loss Adjustment Expense Reserves Member [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	[5]	345	396
Global Structured Finance [Member] | Commercial Mortgage Backed Securities [Member]
Variable Interest Entity [Line Items]
Maximum Exposure to Loss		52	94
Global Structured Finance [Member] | Commercial Mortgage Backed Securities [Member] | Investments [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	[1]	0	0
Global Structured Finance [Member] | Commercial Mortgage Backed Securities [Member] | Premiums Receivable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	[2]	0	0
Global Structured Finance [Member] | Commercial Mortgage Backed Securities [Member] | Insurance Loss Recoverable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	[3]	0	0
Global Structured Finance [Member] | Commercial Mortgage Backed Securities [Member] | Unearned Premium Revenue [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	[4]	0	0
Global Structured Finance [Member] | Commercial Mortgage Backed Securities [Member] | Loss And Loss Adjustment Expense Reserves Member [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	[5]	0	0
Global Structured Finance [Member] | Consumer Asset Backed [Member]
Variable Interest Entity [Line Items]
Maximum Exposure to Loss		560	981
Global Structured Finance [Member] | Consumer Asset Backed [Member] | Investments [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	[1]	0	0
Global Structured Finance [Member] | Consumer Asset Backed [Member] | Premiums Receivable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	[2]	3	4
Global Structured Finance [Member] | Consumer Asset Backed [Member] | Insurance Loss Recoverable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	[3]	1	1
Global Structured Finance [Member] | Consumer Asset Backed [Member] | Unearned Premium Revenue [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	[4]	2	3
Global Structured Finance [Member] | Consumer Asset Backed [Member] | Loss And Loss Adjustment Expense Reserves Member [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	[5]	12	10
Global Structured Finance [Member] | Corporate Asset Backed [Member]
Variable Interest Entity [Line Items]
Maximum Exposure to Loss		1,338	1,645
Global Structured Finance [Member] | Corporate Asset Backed [Member] | Investments [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	[1]	0	0
Global Structured Finance [Member] | Corporate Asset Backed [Member] | Premiums Receivable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	[2]	9	13
Global Structured Finance [Member] | Corporate Asset Backed [Member] | Insurance Loss Recoverable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	[3]	858	0
Global Structured Finance [Member] | Corporate Asset Backed [Member] | Unearned Premium Revenue [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	[4]	10	14
Global Structured Finance [Member] | Corporate Asset Backed [Member] | Loss And Loss Adjustment Expense Reserves Member [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	[5]	0	0
Global Public Finance [Member]
Variable Interest Entity [Line Items]
Maximum Exposure to Loss		2,231	2,524
Global Public Finance [Member] | Investments [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	[1]	0	0
Global Public Finance [Member] | Premiums Receivable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	[2]	9	10
Global Public Finance [Member] | Insurance Loss Recoverable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	[3]	0	0
Global Public Finance [Member] | Unearned Premium Revenue [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	[4]	12	14
Global Public Finance [Member] | Loss And Loss Adjustment Expense Reserves Member [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	[5]	$ 0	$ 0

[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.