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Investments (Investment Securities Whose Book Value Exceeded Market Value by More than 5%) (Details) (USD $) In Millions, unless otherwise specified | 6 Months Ended
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Jun. 30, 2011
Number of Securities | 254
Book Value | $ 3,108
Fair Value | 2,574
5% to 15% [Member] |
Number of Securities | 96
Book Value | 2,124
Fair Value | 1,959
Percentage Book Value Exceeded Market Value, Minimum | 5.00%
Percentage Book Value Exceeded Market Value, Maximum | 15.00%
16% to 25% [Member] |
Number of Securities | 50
Book Value | 444
Fair Value | 357
Percentage Book Value Exceeded Market Value, Minimum | 16.00%
Percentage Book Value Exceeded Market Value, Maximum | 25.00%
26% to 50% [Member] |
Number of Securities | 66
Book Value | 292
Fair Value | 187
Percentage Book Value Exceeded Market Value, Minimum | 26.00%
Percentage Book Value Exceeded Market Value, Maximum | 50.00%
Greater than 50% [Member] |
Number of Securities | 42
Book Value | 248
Fair Value | $ 71
Percentage Book Value Exceeded Market Value, Minimum | 50.00%

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- Definition Disclosure regarding factors used to determine that the impairment of securities not categorized as trading securities (hence equal to available for sale and held to maturity securities) where cost exceeds fair value. This item contains disclosure of the number of investment positions in the investments determined to be temporarily impaired. + Details
Name: | mbi_InvestmentSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions
Namespace Prefix: | mbi
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition Investments Securities In Unrealized Loss Position For Continuous Twelve Month Period or Longer At Fair Value + Details
Name: | mbi_InvestmentsSecuritiesInUnrealizedLossPositionForContinuousTwelveMonthPeriodOrLongerAtFairValue
Namespace Prefix: | mbi
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Investments Securities In Unrealized Loss Position For Continuous Twelve Month Period or Longer Book Value + Details
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Namespace Prefix: | mbi
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Percentage Book Value Exceeded Market Value + Details
Name: | mbi_PercentageBookValueExceededMarketValue
Namespace Prefix: | mbi
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage Book Value Exceeded Market Value, Maximum + Details
Name: | mbi_PercentageBookValueExceededMarketValueMaximum
Namespace Prefix: | mbi
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage Book Value Exceeded Market Value, Minimum + Details
Name: | mbi_PercentageBookValueExceededMarketValueMinimum
Namespace Prefix: | mbi
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration