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Variable Interest Entities (Summary Of Nonconsolidated VIEs Assets And Liabilities) (Detail) - USD ($)
$ in Millions

Mar. 31, 2018

Dec. 31, 2017

Variable Interest Entity [Line Items]

Insurance loss recoverable

$ 530

$ 511

Loss and loss adjustment expense reserves

1,006

979

Global Structured Finance [Member]

Variable Interest Entity [Line Items]

VIE Assets

14,337

14,939

Maximum Exposure to Loss

6,107

6,461

Investments

^[1]

20

19

Premiums receivable

^[2]

36

39

Insurance loss recoverable

^[3]

160

173

Unearned premium revenue

^[4]

35

37

Loss and loss adjustment expense reserves

^[5]

389

406

Global Structured Finance [Member] | Collateralized Debt Obligations [Member]

Variable Interest Entity [Line Items]

VIE Assets

0

0

Maximum Exposure to Loss

0

0

Investments

^[1]

0

0

Premiums receivable

^[2]

0

0

Unearned premium revenue

0

^[4]

0

Loss and loss adjustment expense reserves

^[5]

0

0

Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member]

Variable Interest Entity [Line Items]

VIE Assets

6,783

7,295

Maximum Exposure to Loss

3,573

3,741

Investments

^[1]

20

19

Premiums receivable

^[2]

22

22

Insurance loss recoverable

^[3]

159

172

Unearned premium revenue

^[4]

20

20

Loss and loss adjustment expense reserves

^[5]

379

396

Global Structured Finance [Member] | Commercial Mortgage Backed Securities [Member]

Variable Interest Entity [Line Items]

VIE Assets

205

216

Maximum Exposure to Loss

82

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

5,089

5,010

Maximum Exposure to Loss

926

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

2,260

2,418

Maximum Exposure to Loss

1,526

1,645

Investments

^[1]

0

0

Premiums receivable

^[2]

11

13

Insurance loss recoverable

0

^[3]

0

Unearned premium revenue

^[4]

12

14

Global Public Finance [Member]

Variable Interest Entity [Line Items]

VIE Assets

16,065

15,568

Maximum Exposure to Loss

2,493

2,524

Premiums receivable

^[2]

10

10

Unearned premium revenue

^[4]

13

14

Total Insurance [Member]

Variable Interest Entity [Line Items]

VIE Assets

30,402

30,507

Maximum Exposure to Loss

8,600

8,985

Investments

^[1]

20

19

Premiums receivable

^[2]

46

49

Insurance loss recoverable

^[3]

160

173

Unearned premium revenue

^[4]

48

51

Loss and loss adjustment expense reserves

^[5]

$ 389

$ 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.

X

- Definition

Carrying amount as of the balance sheet date of paid insurance policy claims and LAE expense expected to be recovered.

+ References

No definition available.

+ Details

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Namespace Prefix:	mbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Sum of the carrying amounts as of the balance sheet date of all investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(h))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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

X

- Definition

The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (cc)
-URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437

+ Details

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

X

- Definition

The carrying amount as of the balance sheet date due the entity from (a) agents and insureds, (b) uncollected premiums and (c) others, net of the allowance for doubtful accounts.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.(a),5)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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

X

- Definition

Carrying amount of premiums written on insurance contracts that have not been earned as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(13)(a)(2))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 235
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-16(Column D))
-URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574960-122915

+ Details

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

X

- Definition

The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph b
-URI http://asc.fasb.org/extlink&oid=77990344&loc=d3e5728-111685

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=77990344&loc=d3e5747-111685

+ Details

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

X

- 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

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

X

- Definition

Carrying amount of the assets in the reporting entity's statement of financial position that relate to the reporting entity's variable interest in the Variable Interest Entity (VIE), where the reporting entity is not the VIE's primary beneficiary.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph a
-URI http://asc.fasb.org/extlink&oid=77990344&loc=d3e5728-111685

+ Details

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

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_UnderlyingAssetClassAxis=us-gaap_CollateralizedDebtObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_UnderlyingAssetClassAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_UnderlyingAssetClassAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_UnderlyingAssetClassAxis=mbi_ConsumerAssetBackedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_UnderlyingAssetClassAxis=mbi_CorporateAssetBackedMember
Namespace Prefix:
Data Type:	na
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Period Type:

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- Details

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Namespace Prefix:
Data Type:	na
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- Details

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