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Fair Value Of Financial Instruments (Quantitative Information Regarding Significant Unobservable Inputs For Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) (USD $) In Millions, unless otherwise specified | 9 Months Ended
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Sep. 30, 2012
Commercial Mortgage-Backed [Member] | Recovery Rates [Member] | Minimum [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 20.00%
Commercial Mortgage-Backed [Member] | Recovery Rates [Member] | Maximum [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 90.00%
Commercial Mortgage-Backed [Member] | Recovery Rates [Member] | Weighted Average [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 51.00%
Commercial Mortgage-Backed [Member] | Nonperformance Risk [Member] | Minimum [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 8.00%
Commercial Mortgage-Backed [Member] | Nonperformance Risk [Member] | Maximum [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 62.00%
Commercial Mortgage-Backed [Member] | Nonperformance Risk [Member] | Weighted Average [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 57.00%
Commercial Mortgage-Backed [Member] | Weighted Average Life [Member] | Minimum [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range (in years) | 0.3
Commercial Mortgage-Backed [Member] | Weighted Average Life [Member] | Maximum [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range (in years) | 5.9
Commercial Mortgage-Backed [Member] | Weighted Average Life [Member] | Weighted Average [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range (in years) | 4.6
Commercial Mortgage-Backed [Member] | CMBS Spreads [Member] | Minimum [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 1.00%
Commercial Mortgage-Backed [Member] | CMBS Spreads [Member] | Maximum [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 22.00%
Commercial Mortgage-Backed [Member] | CMBS Spreads [Member] | Weighted Average [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 12.00%
Multi-Sector CDO [Member] | Nonperformance Risk [Member] | Minimum [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 63.00%
Multi-Sector CDO [Member] | Nonperformance Risk [Member] | Maximum [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 63.00%
Multi-Sector CDO [Member] | Nonperformance Risk [Member] | Weighted Average [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 63.00%
Other [Member] | Recovery Rates [Member] | Minimum [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 42.00%
Other [Member] | Recovery Rates [Member] | Maximum [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 75.00%
Other [Member] | Recovery Rates [Member] | Weighted Average [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 47.00%
Other [Member] | Nonperformance Risk [Member] | Minimum [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 8.00%
Other [Member] | Nonperformance Risk [Member] | Maximum [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 63.00%
Other [Member] | Nonperformance Risk [Member] | Weighted Average [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 50.00%
Other [Member] | Weighted Average Life [Member] | Minimum [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range (in years) | 0.2
Other [Member] | Weighted Average Life [Member] | Maximum [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range (in years) | 19
Other [Member] | Weighted Average Life [Member] | Weighted Average [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range (in years) | 3.1
Credit Derivatives [Member] | Commercial Mortgage-Backed [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Valuation Techniques | BET Model
Fair Value | $ 1,533
Credit Derivatives [Member] | Multi-Sector CDO [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Valuation Techniques | Direct Price Model
Fair Value | 628
Credit Derivatives [Member] | Other [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Valuation Techniques | BET Model
Fair Value | 1,156
Loans Receivable [Member] | Variable Interest Entity [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Valuation Techniques | Quoted market prices adjusted for financial guarantees provided to VIE obligations
Fair Value | 1,892
Loans Receivable [Member] | Variable Interest Entity [Member] | Impact Of Financial Guarantee [Member] | Minimum [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 0.00%
Loans Receivable [Member] | Variable Interest Entity [Member] | Impact Of Financial Guarantee [Member] | Maximum [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 22.00%
Loans Receivable [Member] | Variable Interest Entity [Member] | Impact Of Financial Guarantee [Member] | Weighted Average [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 4.00%
Loan Repurchase Commitments [Member] | Variable Interest Entity [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Valuation Techniques | Discounted cash flow
Fair Value | 1,051
Loan Repurchase Commitments [Member] | Variable Interest Entity [Member] | Recovery Rates [Member] | Minimum [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 9.00%
Loan Repurchase Commitments [Member] | Variable Interest Entity [Member] | Recovery Rates [Member] | Maximum [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 96.00%
Loan Repurchase Commitments [Member] | Variable Interest Entity [Member] | Recovery Rates [Member] | Weighted Average [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 43.00%
Loan Repurchase Commitments [Member] | Variable Interest Entity [Member] | Breach Rates [Member] | Minimum [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 65.00%
Loan Repurchase Commitments [Member] | Variable Interest Entity [Member] | Breach Rates [Member] | Maximum [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 94.00%
Loan Repurchase Commitments [Member] | Variable Interest Entity [Member] | Breach Rates [Member] | Weighted Average [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 78.00%
Variable Interest Entity Notes [Member] | Variable Interest Entity [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Valuation Techniques | Quoted market prices of VIE assets adjusted for financial guarantees provided
Fair Value, liabilities | $ 1,885
Variable Interest Entity Notes [Member] | Variable Interest Entity [Member] | Impact Of Financial Guarantee [Member] | Minimum [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 0.00%
Variable Interest Entity Notes [Member] | Variable Interest Entity [Member] | Impact Of Financial Guarantee [Member] | Maximum [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 20.00%
Variable Interest Entity Notes [Member] | Variable Interest Entity [Member] | Impact Of Financial Guarantee [Member] | Weighted Average [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Range Percentage | 6.00%

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- Definition Fair Value Disclosure Range In Years + References No definition available. + Details
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Balance Type: | na
Period Type: | instant

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- Definition Fair Value Measurement Range Percentage + References No definition available. + Details
Name: | mbi_FairValueMeasurementRangePercentage
Namespace Prefix: | mbi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Valuation technique + References No definition available. + Details
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Namespace Prefix: | mbi_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition This element represents an asset measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 157 -Paragraph 32 -Subparagraph c -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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Balance Type: | debit
Period Type: | instant

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- Definition This element represents a liability measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant