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Investments - Summary of Sources of Collateral Received and Various Investment Agreement in which Collateral Pledged (Detail) (USD $) In Thousands, unless otherwise specified | Mar. 31, 2013 | Dec. 31, 2012
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Cash and Securities Pledged Directly from Investment Portfolio [Member] |  |
Derivative [Line Items] |  |
Fair Value of Cash and Underlying Securities | $ 609,439 | $ 646,663
Fair Value of Cash and Securities Pledged to Investment and Repurchase Agreement Counterparties | 376,014 | 375,412
Fair Value of Cash and Securities Pledged to Derivative Counterparties | 233,425 | 271,251
Cash and Securities Pledged from its Derivative Counterparties [Member] |  |
Derivative [Line Items] |  |
Fair Value of Cash and Underlying Securities |  |
Fair Value of Cash and Securities Pledged to Investment and Repurchase Agreement Counterparties |  |
Fair Value of Cash and Securities Pledged to Derivative Counterparties |  |

X
- Definition Fair value of cash and securities pledged to derivative counterparties. + References No definition available. + Details
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Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
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X
- Definition Fair value of cash and securities pledged to investment and repurchase agreement counterparties. + References No definition available. + Details
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Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
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X
- Definition Total fair value of cash and underlying securities pledged. + References No definition available. + Details
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Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration