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Loss And Loss Adjustment Expense Reserves (Loss And LAE Activity) (Narrative) (Detail) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2017 | Mar. 31, 2016 | Dec. 31, 2016
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Policyholder (Benefits) And Claims Incurred Net | $ 94 | $ 22 |
Weighted average risk-free rate used to discount claim liability | 2.14% |  |
Insurance loss recoverable | $ 486 |  | $ 504
Loss and loss adjustment expense reserves | 559 |  | 541
Reinsurance Receivables Case Basis | 6 |  | 6
Second Lien Residential Mortgage Backed Securities And Reserves [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Policyholder (Benefits) And Claims Incurred Net | 23 | 12 |
Lae [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Policyholder (Benefits) And Claims Incurred Net | 12 | $ 8 |
Loss and loss adjustment expense reserves | 62 |  | $ 60
Ineligible Mortgage Loans [Member] | Second Lien Residential Mortgage Backed Securities And Reserves [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Contract Claims On Ineligible Mortgage Loans Inception To Date Incurred Loss On Transactions | $ 440 |  |

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- Definition Contract Claims On Ineligible Mortgage Loans Inception To Date Incurred Loss On Transactions + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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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
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Period Type: | instant

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- Definition Loss And Loss Adjustment Expense Reserves [Line Items] + References No definition available. + Details
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- 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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Balance Type: | na
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- 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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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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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 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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Balance Type: | debit
Period Type: | duration

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- Definition The sum for all reinsurance arrangements of the known and estimated amount recoverable as of the balance sheet date from a given reinsurer for claims paid or incurred by the ceding insurer and associated claims settlement expenses for each reinsurance arrangement (or, the sum of all cases determined individually, probably applying different assumptions to each arrangement). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21412-158489 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21409-158489 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.6) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Details
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- Details
Name: | mbi_ClaimLiabilityAxis=mbi_LaeMember
Namespace Prefix: |
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Balance Type: |
Period Type: |

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- Details
Name: | mbi_ClaimLiabilityAxis=mbi_IneligibleMortgageLoansMember
Namespace Prefix: |
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Balance Type: |
Period Type: |