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Insurance Premiums (Narrative) (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended |
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Insurance Premiums [Abstract] |  |  |
Premiums receivable | $ 875 us-gaap_PremiumsReceivableAtCarryingValue | $ 1,051 us-gaap_PremiumsReceivableAtCarryingValue | $ 1,228 us-gaap_PremiumsReceivableAtCarryingValue
Weighted average risk-free rate | 2.80%us-gaap_FinancialGuaranteeInsuranceContractsPremiumReceivedOverContractPeriodPremiumReceivableWeightedAverageRiskFreeDiscountRate | 2.90%us-gaap_FinancialGuaranteeInsuranceContractsPremiumReceivedOverContractPeriodPremiumReceivableWeightedAverageRiskFreeDiscountRate |
Weighted average expected collection term of the premiums receivable, years | 9 years 2 months 4 days | 9 years 2 months 15 days |
Reinsurance premiums payable | $ 56 us-gaap_ReinsurancePayable | $ 51 us-gaap_ReinsurancePayable |

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- Details
Name: | mbi_InsurancePremiumsAbstract
Namespace Prefix: | mbi_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Weighted average period over which premium receivable for financial guarantee insurance contracts is collected where premium is received over the contract period, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 50 -Paragraph 3 -Subparagraph (a)(5) -URI http://asc.fasb.org/extlink&oid=6933075&loc=SL5748726-161290 + Details
Name: | us-gaap_FinancialGuaranteeInsuranceContractsPremiumReceivedOverContractPeriodPremiumReceivableWeightedAverageCollectionPeriod1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition This element represents the weighted average risk free rate used to discount the premiums expected to be collected for financial guarantee insurance contracts where the premium is to be received over the contract period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 50 -Paragraph 3 -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=6933075&loc=SL5748726-161290 + Details
Name: | us-gaap_FinancialGuaranteeInsuranceContractsPremiumReceivedOverContractPeriodPremiumReceivableWeightedAverageRiskFreeDiscountRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- 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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 5 -Article 7 + Details
Name: | us-gaap_PremiumsReceivableAtCarryingValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The carrying amount as of the balance sheet date of the known and estimated amounts owed to insurers under reinsurance treaties or other arrangements. + 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 210.7-03.13(a)(3),(c)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 13 -Subparagraph a(3) -Article 7 + Details
Name: | us-gaap_ReinsurancePayable
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant