Variable Interest Entities (Narrative) (Detail) $ in Millions	6 Months Ended
Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Variable Interest Entity [Line Items]
Carrying amounts of assets	$ 2,200	$ 3,200
Carrying amounts of liabilities	$ 2,200	$ 2,300
Variable Interest Entity Primary Beneficiary [Member] \| Structured Finance And International Insurance [Member]
Variable Interest Entity [Line Items]
Number of variable interest entities consolidated	0
Variable Interest Entity Primary Beneficiary [Member] \| Structured Finance And International Insurance [Member] \| Zohar Funds Bankruptcy Cases Debtors [Member]
Variable Interest Entity [Line Items]
Net realized gains (losses) related to deconsolidation	$ (93)
Number of variable interest entities deconsolidated	2

Represents the number of VIEs consolidated during the period.

No definition available.

Name:	mbi_NumberOfVariableInterestEntitiesConsolidated
Namespace Prefix:	mbi_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of VIEs deconsolidated during the period.

+ References

No definition available.

+ Details

Name:	mbi_NumberOfVariableInterestEntitiesDeconsolidated
Namespace Prefix:	mbi_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of the gain (loss) recognized by the parent and included in its attributable portion of net income for the period due to deconsolidation of a subsidiary or derecognition of a group of assets. The gain (loss) recognized and included in the net income attributable to the parent for the period is generally computed as the difference between: (a) the aggregate of: (1) the fair value of any consideration received; (2) the fair value of any retained noncontrolling investment in the former subsidiary at the date the subsidiary was deconsolidated; and (3) the carrying amount of any noncontrolling interest in the former subsidiary (including any accumulated other comprehensive income attributable to the noncontrolling interest) at the date the subsidiary was deconsolidated and (b) the carrying amount of the former subsidiary's assets and liabilities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 1B
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84161450&loc=SL4582445-111684

+ Details

Name:	us-gaap_DeconsolidationGainOrLossAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The carrying amount of the consolidated Variable Interest Entity's assets included in the reporting entity's statement of financial position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (bb)
-URI http://asc.fasb.org/extlink&oid=77990344&loc=d3e5710-111685

+ Details

Name:	us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The carrying amount of the consolidated Variable Interest Entity's liabilities included in the reporting entity's statement of financial position.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_VariableInterestEntityLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

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Namespace Prefix:
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Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=mbi_InternationalAndStructuredFinanceInsuranceMember
Namespace Prefix:
Data Type:	na
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- Details

Name:	us-gaap_MajorCustomersAxis=mbi_ZoharFundsBankruptcyCasesDebtorsMember
Namespace Prefix:
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