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Fair Value Of Financial Instruments (Quantitative Information Regarding The Significant Unobservable Inputs For Certain Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Detail) (USD $) In Millions, unless otherwise specified | Sep. 30, 2014 | Dec. 31, 2013
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Other Derivative Liabilities [Member] | Discounted cash flow [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Fair value, liabilities | $ 38 |  |
Discounted Cash Flows [Member] | Other Derivative Liabilities [Member] | Minimum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range dollars | 0 |  |
Discounted Cash Flows [Member] | Other Derivative Liabilities [Member] | Maximum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range dollars | 83 |  |
Discounted Cash Flows [Member] | Other Derivative Liabilities [Member] | Weighted Average [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range dollars | 42 | [1] |
Credit Derivatives [Member] | Commercial Mortgage Backed Securities [Member] | BET Model [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Fair value, liabilities | 263 |  | 1,050
Credit Derivatives [Member] | Multi Sector C D O [Member] | Direct Price Model [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Fair value, liabilities | 11 |  | 12
Credit Derivatives [Member] | Other Derivative Liabilities [Member] | BET And Dual Default Valuation Models [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Fair value, liabilities | 10 |  | 85
Credit Derivatives [Member] | Recovery Rates [Member] | Commercial Mortgage Backed Securities [Member] | Minimum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 25.00% |  | 25.00%
Credit Derivatives [Member] | Recovery Rates [Member] | Commercial Mortgage Backed Securities [Member] | Maximum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 90.00% |  | 90.00%
Credit Derivatives [Member] | Recovery Rates [Member] | Commercial Mortgage Backed Securities [Member] | Weighted Average [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 74.00% |  | 60.00%
Credit Derivatives [Member] | Recovery Rates [Member] | Other Derivative Liabilities [Member] | Minimum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 42.00% |  | 42.00%
Credit Derivatives [Member] | Recovery Rates [Member] | Other Derivative Liabilities [Member] | Maximum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 45.00% |  | 90.00%
Credit Derivatives [Member] | Recovery Rates [Member] | Other Derivative Liabilities [Member] | Weighted Average [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 45.00% |  | 45.00%
Credit Derivatives [Member] | Nonperformance Risk [Member] | Commercial Mortgage Backed Securities [Member] | Minimum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 13.00% |  | 8.00%
Credit Derivatives [Member] | Nonperformance Risk [Member] | Commercial Mortgage Backed Securities [Member] | Maximum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 24.00% |  | 57.00%
Credit Derivatives [Member] | Nonperformance Risk [Member] | Commercial Mortgage Backed Securities [Member] | Weighted Average [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 22.00% |  | 25.00%
Credit Derivatives [Member] | Nonperformance Risk [Member] | Multi Sector C D O [Member] | Minimum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 51.00% |  | 57.00%
Credit Derivatives [Member] | Nonperformance Risk [Member] | Multi Sector C D O [Member] | Maximum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 51.00% |  | 57.00%
Credit Derivatives [Member] | Nonperformance Risk [Member] | Multi Sector C D O [Member] | Weighted Average [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 51.00% |  | 57.00%
Credit Derivatives [Member] | Nonperformance Risk [Member] | Other Derivative Liabilities [Member] | Minimum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 40.00% |  | 13.00%
Credit Derivatives [Member] | Nonperformance Risk [Member] | Other Derivative Liabilities [Member] | Maximum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 50.00% |  | 54.00%
Credit Derivatives [Member] | Nonperformance Risk [Member] | Other Derivative Liabilities [Member] | Weighted Average [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 48.00% |  | 25.00%
Credit Derivatives [Member] | Weighted Average Life [Member] | Commercial Mortgage Backed Securities [Member] | Minimum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Weighted Average (in years) | 1.6 |  | 1.1
Credit Derivatives [Member] | Weighted Average Life [Member] | Commercial Mortgage Backed Securities [Member] | Maximum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Weighted Average (in years) | 3.1 |  | 28
Credit Derivatives [Member] | Weighted Average Life [Member] | Commercial Mortgage Backed Securities [Member] | Weighted Average [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Weighted Average (in years) | 2.5 |  | 3.3
Credit Derivatives [Member] | Weighted Average Life [Member] | Other Derivative Liabilities [Member] | Minimum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Weighted Average (in years) | 0.5 |  | 0.2
Credit Derivatives [Member] | Weighted Average Life [Member] | Other Derivative Liabilities [Member] | Maximum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Weighted Average (in years) | 8.1 |  | 8.7
Credit Derivatives [Member] | Weighted Average Life [Member] | Other Derivative Liabilities [Member] | Weighted Average [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Weighted Average (in years) | 1.4 |  | 2.3
Credit Derivatives [Member] | Commercial Mortgage Backed Securities Spreads [Member] | Commercial Mortgage Backed Securities [Member] | Minimum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 0.00% |  | 1.00%
Credit Derivatives [Member] | Commercial Mortgage Backed Securities Spreads [Member] | Commercial Mortgage Backed Securities [Member] | Maximum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 37.00% |  | 29.00%
Credit Derivatives [Member] | Commercial Mortgage Backed Securities Spreads [Member] | Commercial Mortgage Backed Securities [Member] | Weighted Average [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 11.00% |  | 13.00%
Loans Receivable [Member] | Variable Interest Entity Primary Beneficiary [Member] | Quoted market prices adjusted for financial guarantees provided to VIE obligations [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Fair Value, assets | 1,495 |  | 1,612
Loans Receivable [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Minimum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 0.00% |  | 0.00%
Loans Receivable [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Maximum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 8.00% |  | 17.00%
Loans Receivable [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Weighted Average [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 2.00% |  | 3.00%
Loan Repurchase Commitments [Member] | Variable Interest Entity Primary Beneficiary [Member] | Discounted cash flow [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Fair Value, assets | 365 |  | 359
Variable Interest Entity Notes [Member] | Variable Interest Entity Primary Beneficiary [Member] | Quoted market prices of VIE assets adjusted for financial guarantees provided [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Fair value, liabilities | $ 746 |  | $ 940
Variable Interest Entity Notes [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Minimum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 0.00% |  | 0.00%
Variable Interest Entity Notes [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Maximum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 35.00% |  | 25.00%
Variable Interest Entity Notes [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Weighted Average [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 16.00% |  | 12.00%
[1] | Midpoint of cash flows are used for the weighted average.

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- Definition Fair Value Disclosure, Weighted Average (In Years) + References No definition available. + Details
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- Definition This element represents the fair value measurement range dollars of the Company's assets and liabilities measured at fair value. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition This element represents the fair value measurement range percentage of the Company's assets and liabilities measured at fair value. + References No definition available. + Details
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Namespace Prefix: | mbi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Details
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- Definition Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + 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=25499696&loc=d3e19207-110258 + Details
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- Definition Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + 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=25499696&loc=d3e19207-110258 + Details
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Balance Type: | credit
Period Type: | instant