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Derivative Instruments - Summarize Information by Major Category of CDS Contracts (Detail) (USD $) In Thousands, unless otherwise specified |  |  | 12 Months Ended |  | 9 Months Ended |  | 12 Months Ended |  | 9 Months Ended |  | 12 Months Ended |  | 9 Months Ended |
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Sep. 30, 2013 | Dec. 31, 2012 | Dec. 31, 2012 Predecessor Ambac [Member] Transactions | Dec. 31, 2012 Predecessor Ambac [Member] Credit Derivatives [Member] | Sep. 30, 2013 Successor Ambac [Member] Transactions | Sep. 30, 2013 Successor Ambac [Member] Credit Derivatives [Member] | Dec. 31, 2012 CLO [Member] Predecessor Ambac [Member] Transactions | Dec. 31, 2012 CLO [Member] Predecessor Ambac [Member] Credit Derivatives [Member] | Sep. 30, 2013 CLO [Member] Successor Ambac [Member] Transactions | Sep. 30, 2013 CLO [Member] Successor Ambac [Member] Credit Derivatives [Member] | Dec. 31, 2012 Other [Member] Predecessor Ambac [Member] Transactions | Dec. 31, 2012 Other [Member] Predecessor Ambac [Member] Credit Derivatives [Member] | Sep. 30, 2013 Other [Member] Successor Ambac [Member] Transactions | Sep. 30, 2013 Other [Member] Successor Ambac [Member] Credit Derivatives [Member]
Summary of information by major category of CDS contracts |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of CDS transactions |  |  | 51 |  | 26 |  | 30 |  | 8 |  | 21 |  | 18 |
Remaining expected weighted-average life of obligations (in years) |  |  | 3 years 7 months 6 days |  | 4 years 4 months 24 days |  | 2 years 2 months 12 days |  | 2 years 3 months 18 days |  | 5 years 2 months 12 days |  | 5 years 3 months 18 days |
Gross principal notional outstanding | $ 5,207,364 | $ 11,281,777 | $ 11,281,777 |  | $ 5,207,364 |  | $ 6,155,767 |  | $ 1,609,378 |  | $ 5,126,010 |  | $ 3,597,986 |
Net derivative liabilities at fair value |  |  |  | $ (213,585) |  | $ (202,653) |  | $ (34,645) |  | $ (10,201) |  | $ (178,940) |  | $ (192,452)

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition Average remaining period until maturity of the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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- Definition Fair value of financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, elected not to be offset against a right to receive collateral under a master netting arrangement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=20225523&loc=SL20225862-175312 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=7476318&loc=d3e41678-113959 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=28370219&loc=SL20226011-175313 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=28370219&loc=SL20226006-175313 + Details
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- Definition The number of derivative instruments of a particular group held by the entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5580258-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5579245-113959 + Details
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