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Loss And Loss Adjustment Expense Reserves (Loss And LAE Activity) (Narrative) (Detail) (USD $) | 3 Months Ended | 9 Months Ended |  | 9 Months Ended |  |  |  |  |  |  |  |
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Sep. 30, 2013 | Sep. 30, 2012 | Sep. 30, 2013 | Sep. 30, 2012 | Sep. 30, 2013 U S Public Finance Insurance [Member] | Sep. 30, 2013 Abs Cdo Portfolio [Member] | Sep. 30, 2013 Insured Issues Designated As Classified List [Member] Variable Interest Entity Primary Beneficiary [Member] issue | Sep. 30, 2013 Rmbs And Heloc And Ces [Member] | Sep. 30, 2013 Rmbs And Heloc And Ces [Member] Ineligible Loan Recoveries [Member] | Sep. 30, 2013 Second Lien Residential Mortgage Backed Securities And Reserves [Member] | Sep. 30, 2013 Second Lien Residential Mortgage Backed Securities And Reserves [Member] Ineligible Loan Recoveries [Member] | Sep. 30, 2013 Financial Guarantee [Member] | Sep. 30, 2013 Financial Guarantee [Member] Variable Interest Entity Primary Beneficiary [Member] | Sep. 30, 2013 Loss And Lae Reserves [Member] | Dec. 31, 2012 Loss And Lae Reserves [Member] | Sep. 30, 2013 Recoveries On Unpaid Losses [Member] | Dec. 31, 2012 Recoveries On Unpaid Losses [Member] | Sep. 30, 2013 Insurance Loss Recoverable [Member] | Dec. 31, 2012 Insurance Loss Recoverable [Member] | Sep. 30, 2013 Insurance Loss Recoverable [Member] Classified [Member] | Dec. 31, 2012 Insurance Loss Recoverable [Member] Classified [Member]
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
ABS CDO exposure decrease since December 31,2007 |  |  |  |  |  | 96.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate ABS CDO Loss And LAE Reserves |  |  |  |  |  |  |  |  |  |  |  | $ 87,000,000 | $ 196,000,000 |  |  |  |  |  |  |  |
Policyholder (Benefits) And Claims Incurred Net | 98,000,000 | 171,000,000 | 92,000,000 | 330,000,000 |  | (47,000,000) |  |  |  | (92,000,000) |  |  |  |  |  |  |  |  |  |  |
Elimination Of Loss And LAE Expense As Result Of Consolidating VIE |  |  |  |  |  | 39,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reduction of excess interest cash flows from securitizations |  |  |  |  |  |  |  | 139,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Losses and loss adjustment expense payments, net of reinsurance |  |  | 372,000,000 |  | 134,000,000 |  |  | 166,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Decrease in insurance loss recoverable related to paid losses |  |  | 2,900,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reserves for expected future claim payments |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,000,000,000 | 1,200,000,000 |  |  |  |  |  |
Expected recoveries of future payments |  |  |  |  |  |  |  |  |  |  |  |  |  | 326,000,000 | 332,000,000 | 326,000,000 | 332,000,000 |  |  |  |
Number of insured issues reviewed |  |  |  |  |  |  | 11 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted average risk-free rate used to discount claim liability |  |  | 2.11% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total estimated insurance recoveries |  |  |  |  |  |  |  |  |  |  | 1,100,000,000 |  |  |  |  |  |  |  |  |  |
Insurance claim loss recoverable |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 737,000,000 | 3,610,000,000
Insurance loss recoverable |  |  |  |  |  |  |  |  |  |  | 18,000,000 |  |  |  |  |  |  | 742,000,000 | 3,648,000,000 | 742,000,000 | 3,648,000,000
Loan repurchase commitments |  |  |  |  |  |  |  |  |  |  | 1,100,000,000 |  |  |  |  |  |  |  |  |  |
Loss and loss adjustment expense collections, net of reinsurance |  |  | 2,900,000,000 |  |  |  |  | 2,800,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Losses and LAE related to actual and expected payments |  |  | 205,000,000 |  | 86,000,000 |  |  |  |  | 85,000,000 |  |  |  |  |  |  |  |  |  |  |
Change in recoveries of actual and expected payments related to ineligible mortgages |  |  |  |  |  |  |  |  | $ 316,000,000 |  |  |  |  |  |  |  |  |  |  |  |

X
- Definition ABS CDO exposure decrease. + References No definition available. + Details
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Namespace Prefix: | mbi_
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Balance Type: | na
Period Type: | duration

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- Definition Aggregate ABS CDO loss and LAE reserves. + References No definition available. + Details
Name: | mbi_AggregateAbsCdoLossAndLaeReserves
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Change In Recoveries Of Actual And Expected Payments Related To Ineligible Mortgages + References No definition available. + Details
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Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
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Period Type: | duration

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- Definition Elimination Of Loss And LAE Expense As Result Of Consolidating VIEs + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Excess Interest Cash Flows From Securitizations + References No definition available. + Details
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Balance Type: | debit
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- Definition Gross reserve, recoveries on unpaid losses + References No definition available. + Details
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Balance Type: | debit
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- Definition Increase decrease in carrying amount as of the balance sheet date of amounts expected to be recovered under the terms of insurance contracts related to recoveries related to commitments to recoveries on paid claims. + References No definition available. + Details
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Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
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- 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
Name: | mbi_InsuranceLossRecoverable
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The fair value of financial instruments that represent the right of the Trust against the seller/servicer for loans that were not compliant with the representations and warranties of the loan underwriting. The Trust has the right to demand that the transferor replace ineligible loans with a compliant loan or repurchase the non-compliant loan. + References No definition available. + Details
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- Definition Loss And Loss Adjustment Expense Collections Net Of Reinsurance And Payments + References No definition available. + Details
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- Details
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- Definition This element represents losses and loss adjustments expense payments net of reinsurance. + References No definition available. + Details
Name: | mbi_LossesAndLossAdjustmentExpensePaymentsNetOfReinsurance
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
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- Definition Losses And Loss Adjustment Expenses Related To Actual And Expected Payments + References No definition available. + Details
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- Definition Number Of Insured Issues Reviewed + References No definition available. + Details
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- Definition Reserves For Expected Future Claim Payments + References No definition available. + Details
Name: | mbi_ReservesForExpectedFutureClaimPayments
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Total Estimated Insurance Recoveries + References No definition available. + Details
Name: | mbi_TotalEstimatedInsuranceRecoveries
Namespace Prefix: | mbi_
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Balance Type: | debit
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- Definition Carrying amount as of the balance sheet date of amounts expected to be recovered under the terms of insurance contracts. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.17) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 17 -Article 5 + Details
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X
- Definition This element represents the weighted average risk free rate used to discount the claim liability related to financial guarantee insurance contracts. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 9 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6933314&loc=SL5749324-161292 + Details
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- Definition Amount, after effects of policies assumed or ceded, of expense related to the provision for policy benefits and costs incurred. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.5) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 5 -Article 7 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6811-158387 + Details
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