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Fair Value Measurements - Summary of Gains and Losses (Realized and Unrealized) Relating to Level 3 Assets and Liabilities Included in Earnings (Detail) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 5 Months Ended | 3 Months Ended | 4 Months Ended | 9 Months Ended | 3 Months Ended | 5 Months Ended | 3 Months Ended | 4 Months Ended | 9 Months Ended | 3 Months Ended | 5 Months Ended | 3 Months Ended | 4 Months Ended | 9 Months Ended | 3 Months Ended | 5 Months Ended | 3 Months Ended | 4 Months Ended | 9 Months Ended | 3 Months Ended | 5 Months Ended | 3 Months Ended | 4 Months Ended | 9 Months Ended | 3 Months Ended | 5 Months Ended | 3 Months Ended | 4 Months Ended | 9 Months Ended
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Sep. 30, 2013 Successor Ambac [Member] Variable Interest Entity [Member] | Sep. 30, 2013 Successor Ambac [Member] Variable Interest Entity [Member] | Sep. 30, 2012 Predecessor Ambac [Member] Variable Interest Entity [Member] | Apr. 30, 2013 Predecessor Ambac [Member] Variable Interest Entity [Member] | Sep. 30, 2012 Predecessor Ambac [Member] Variable Interest Entity [Member] | Sep. 30, 2013 Realized Gains Or (Losses) and Other Settlements On Credit Derivative Contracts [Member] Successor Ambac [Member] | Sep. 30, 2013 Realized Gains Or (Losses) and Other Settlements On Credit Derivative Contracts [Member] Successor Ambac [Member] | Sep. 30, 2012 Realized Gains Or (Losses) and Other Settlements On Credit Derivative Contracts [Member] Predecessor Ambac [Member] | Apr. 30, 2013 Realized Gains Or (Losses) and Other Settlements On Credit Derivative Contracts [Member] Predecessor Ambac [Member] | Sep. 30, 2012 Realized Gains Or (Losses) and Other Settlements On Credit Derivative Contracts [Member] Predecessor Ambac [Member] | Sep. 30, 2013 Unrealized Gains Or (Losses) On Credit Derivative Contracts [Member] Successor Ambac [Member] | Sep. 30, 2013 Unrealized Gains Or (Losses) On Credit Derivative Contracts [Member] Successor Ambac [Member] | Sep. 30, 2012 Unrealized Gains Or (Losses) On Credit Derivative Contracts [Member] Predecessor Ambac [Member] | Apr. 30, 2013 Unrealized Gains Or (Losses) On Credit Derivative Contracts [Member] Predecessor Ambac [Member] | Sep. 30, 2012 Unrealized Gains Or (Losses) On Credit Derivative Contracts [Member] Predecessor Ambac [Member] | Sep. 30, 2013 Interest Rate Swaps [Member] Successor Ambac [Member] | Sep. 30, 2013 Interest Rate Swaps [Member] Successor Ambac [Member] | Sep. 30, 2012 Interest Rate Swaps [Member] Predecessor Ambac [Member] | Apr. 30, 2013 Interest Rate Swaps [Member] Predecessor Ambac [Member] | Sep. 30, 2012 Interest Rate Swaps [Member] Predecessor Ambac [Member] | Sep. 30, 2013 Other Income or (Loss) [Member] Successor Ambac [Member] | Sep. 30, 2013 Other Income or (Loss) [Member] Successor Ambac [Member] | Sep. 30, 2012 Other Income or (Loss) [Member] Predecessor Ambac [Member] | Apr. 30, 2013 Other Income or (Loss) [Member] Predecessor Ambac [Member] | Sep. 30, 2012 Other Income or (Loss) [Member] Predecessor Ambac [Member] | Sep. 30, 2013 Net Investment Income [Member] Successor Ambac [Member] | Sep. 30, 2013 Net Investment Income [Member] Successor Ambac [Member] | Sep. 30, 2012 Net Investment Income [Member] Predecessor Ambac [Member] | Apr. 30, 2013 Net Investment Income [Member] Predecessor Ambac [Member] | Sep. 30, 2012 Net Investment Income [Member] Predecessor Ambac [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total gains or losses included in earnings for the period |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 467 | $ 863 | $ (22) | $ (33) | $ 17
Total gains or losses included in earnings for the period | 253,815 | (537,477) | 341,578 | 1,146,256 | 462,009 | 1,581 | 7,655 | 2,945 | 3,444 | 9,271 | 29,614 | 74,760 | 24,496 | (63,828) | 3,532 | 5,931 | 24,838 | (1,156) | (28,162) | (46,191) | (213) | (390) | (771) | (496) | 98,952 |  |  |  |  |
Changes in unrealized gains or losses relating to the assets and liabilities still held at the reporting date | $ 253,815 | $ (537,477) | $ 341,578 | $ 1,146,256 | $ 462,490 |  |  |  |  |  | $ 27,053 | $ 40,671 | $ 23,948 | $ (63,840) | $ (4,192) | $ 5,931 | $ 24,838 | $ (1,156) | $ (28,162) | $ (47,419) | $ (213) | $ (390) | $ (771) | $ (496) | $ (1,758) |  |  |  |  |

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- Definition The amount of gains or losses included in earnings attributable to unrealized gains or losses on financial instruments measured on a recurring basis using unobservable inputs that are still held at the reporting date. + References No definition available. + Details
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