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Special Purpose Entities, Including Variable Interest Entities - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified | 3 Months Ended |  | 3 Months Ended |  | 3 Months Ended |  |  | 3 Months Ended | 12 Months Ended
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Mar. 31, 2013 | Mar. 31, 2012 | Dec. 31, 2012 | Mar. 31, 2013 Asset-backed securities and utility obligations [Member] | Mar. 31, 2013 Other assets [Member] | Mar. 31, 2012 Other assets [Member] | Mar. 31, 2013 Sponsored Variable Interest Entities [Member] Transactions | Dec. 31, 2012 Sponsored Variable Interest Entities [Member] | Mar. 31, 2013 Financial guarantee [Member] | Mar. 31, 2012 Financial guarantee [Member] | Mar. 31, 2013 Variable interest entity [Member] | Dec. 31, 2012 Variable interest entity [Member] | Mar. 31, 2013 Consolidated entities [Member] Entity | Dec. 31, 2012 Consolidated entities [Member] Entity
Variable Interest Entities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt outstanding to third parties under secured borrowing transaction |  |  |  |  |  |  |  |  |  |  | $ 10,521 | $ 14,588 |  |
Securities fair value | 218,735 |  | 201,329 |  |  |  |  |  |  |  | 218,735 | 201,329 |  |
Fair value of special purpose entities | 14,230 |  | 14,557 |  |  |  |  |  |  |  |  |  |  |
Change in fair value of special purpose entities |  |  |  |  | (327) | (756) |  |  |  |  |  |  |  |
Number of individual transactions with special purpose entities |  |  |  |  |  |  | 15 |  |  |  |  |  |  |
Number of transactions outstanding with special purpose entities |  |  |  |  |  |  | 3 |  |  |  |  |  |  |
Total principal amount of debt outstanding |  |  |  |  |  |  | 464,495 | 466,938 |  |  |  |  |  |
Weighted average life |  |  |  | 8 years 8 months 12 days |  |  |  |  |  |  |  |  |  |
Earned premiums for issuing the financial guarantee policies on the assets, MTNs and derivative contracts | 30 | 145 |  |  |  |  |  |  |  |  |  |  |  |
Assets sold to special purpose entities | 1,455 | 134 |  |  |  |  |  |  |  |  |  |  |  |
Claims on insurance policies issued to these entities | 0 |  |  |  |  |  |  |  |  |  |  |  |  |
Earned fees for providing other services under its financial guarantee policies |  |  |  |  |  |  |  |  | 2 | 11 |  |  |  |
Amount received under derivative contracts | 93 | 138 |  |  |  |  |  |  |  |  |  |  |  |
Number of consolidated Variable Interest Entities |  |  |  |  |  |  |  |  |  |  |  |  | 18 | 18
Consolidated VIE assets | 16,751,505 |  | 17,841,863 |  |  |  |  |  |  |  |  |  |  |
Consolidated VIE liabilities | $ 16,548,127 |  | $ 17,661,700 |  |  |  |  |  |  |  |  |  |  |
Average rating of assets held by sponsored special purpose entities | No longer than 90 days |  |  |  |  |  |  |  |  |  |  |  |  |

X
- Definition Amount received under derivative contracts entered into with Ambac's sponsored medium term note special purpose entities. + References No definition available. + Details
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Balance Type: | credit
Period Type: | duration

X
- Definition Amounts recovered in respect of previously paid claims on insurance policies issued to Ambac's sponsored special purpose entities. + References No definition available. + Details
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- Definition Assets Transferred To Special Purpose Entities + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition The total principal amount of outstanding debt issued by Ambac's sponsored medium term note special purpose entities. + References No definition available. + Details
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Balance Type: | credit
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- Definition Amount of gain or loss included in earnings arising from financial instruments measured on a recurring basis using unobservable inputs. + References No definition available. + Details
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- Definition Amount of insurance premiums earned related to financial guarantee policies on the assets, debt and derivative contracts of Ambac's sponsored medium term note special purpose entities. + References No definition available. + Details
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- Definition Number of consolidated variable interest entities. + References No definition available. + Details
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- Definition The number of asset transfer transactions executed with Ambac's sponsored medium term note special purpose entities that are still outstanding at the reporting date. + References No definition available. + Details
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- Definition The number of asset transfer transactions executed with Ambac's sponsored medium term note special purpose entities since the inception of those entities. + References No definition available. + Details
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- Definition Fair Value of securities held to collateralize and fund repayment of debt issued through a resecuritization transaction. + References No definition available. + Details
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- Definition The weighted average remaining life of assets held by Ambac's sponsored medium term note special purpose entities at the reporting date. + References No definition available. + Details
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- Definition The weighted average credit rating of assets held by Ambac's sponsored medium term note special purpose entities at the reporting date. + References No definition available. + Details
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- Definition This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents investments accounted for under the equity method of accounting as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 18 -Paragraph 6 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 25 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6903542&loc=d3e32014-111567 + Details
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- Definition Fees and other service revenues recognized in the period that are not otherwise specified in the taxonomy. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1(d)) -URI http://asc.fasb.org/extlink&oid=6880815&loc=d3e20235-122688 + Details
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- Definition Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 20, 22 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 9 + Details
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- Definition The carrying amount of the consolidated Variable Interest Entity's assets included in the reporting entity's statement of financial position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=7880789&loc=d3e5710-111685 + Details
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- Definition The carrying amount of the consolidated Variable Interest Entity's liabilities included in the reporting entity's statement of financial position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=7880789&loc=d3e5710-111685 + Details
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- Details
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