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Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Detail) (USD $) In Thousands, unless otherwise specified | Sep. 30, 2013 Residential Mortgage-Backed Securities [Member] | Sep. 30, 2013 Other Asset-Backed Securities [Member] | Dec. 31, 2012 Predecessor Ambac [Member] | Dec. 31, 2012 Predecessor Ambac [Member] Short-Term [Member] | Dec. 31, 2012 Predecessor Ambac [Member] Fixed Income Securities [Member] | Dec. 31, 2012 Predecessor Ambac [Member] Fixed Income Securities [Member] Municipal Obligations [Member] | Dec. 31, 2012 Predecessor Ambac [Member] Fixed Income Securities [Member] Corporate Obligations [Member] | Dec. 31, 2012 Predecessor Ambac [Member] Fixed Income Securities [Member] U.S. Government Obligations [Member] | Dec. 31, 2012 Predecessor Ambac [Member] Fixed Income Securities [Member] Residential Mortgage-Backed Securities [Member] | Dec. 31, 2012 Predecessor Ambac [Member] Fixed Income Securities [Member] Collateralized Debt Obligations [Member] | Dec. 31, 2012 Predecessor Ambac [Member] Fixed Income Securities [Member] Other Asset-Backed Securities [Member] | Sep. 30, 2013 Successor Ambac [Member] | Sep. 30, 2013 Successor Ambac [Member] Short-Term [Member] | Sep. 30, 2013 Successor Ambac [Member] Fixed Income Securities [Member] | Sep. 30, 2013 Successor Ambac [Member] Fixed Income Securities [Member] Municipal Obligations [Member] | Sep. 30, 2013 Successor Ambac [Member] Fixed Income Securities [Member] Corporate Obligations [Member] | Sep. 30, 2013 Successor Ambac [Member] Fixed Income Securities [Member] U.S. Government Obligations [Member] | Sep. 30, 2013 Successor Ambac [Member] Fixed Income Securities [Member] Residential Mortgage-Backed Securities [Member] | Sep. 30, 2013 Successor Ambac [Member] Fixed Income Securities [Member] Collateralized Debt Obligations [Member] | Sep. 30, 2013 Successor Ambac [Member] Fixed Income Securities [Member] Other Asset-Backed Securities [Member] | Sep. 30, 2013 Successor Ambac [Member] Fixed Income Securities [Member] Foreign Obligations [Member] | Sep. 30, 2013 Successor Ambac [Member] Fixed Income Securities [Member] U.S. Agency Obligations [Member]
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Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Less than 12 Months, Fair Value |  |  | $ 228,442 | $ 1,194 | $ 227,248 | $ 42,503 | $ 69,727 | $ 26,081 | $ 88,504 | $ 253 | $ 180 | $ 2,934,366 | $ 1,298 | $ 2,933,068 | $ 455,719 | $ 962,892 | $ 33,653 | $ 580,747 | $ 37,018 | $ 743,499 | $ 114,986 | $ 4,554
Less than 12 Months, Gross Unrealized Loss | 21,039 | 23,340 | 7,551 |  | 7,551 | 354 | 1,081 | 626 | 5,319 | 168 | 3 | 101,753 | 5 | 101,748 | 26,157 | 24,324 | 1,381 | 21,039 | 224 | 23,340 | 5,206 | 77
12 Months or More, Fair Value |  |  |  |  | 455,626 | 4,303 | 132,916 |  | 116,146 | 13,429 | 188,832 |  |  |  |  |  |  |  |  |  |  |
12 Months or More, Fair Value |  |  | 455,626 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
12 Months or More, Gross Unrealized Loss |  |  |  |  | 70,164 | 29 | 8,036 |  | 15,236 | 360 | 46,503 |  |  |  |  |  |  |  |  |  |  |
12 Months or More, Gross Unrealized Losses |  |  | 70,164 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total, Fair Value |  |  |  |  | 682,874 | 46,806 | 202,643 | 26,081 | 204,650 | 13,682 | 189,012 |  |  |  |  |  |  |  |  |  |  |
Total, Fair Value |  |  | 684,068 | 1,194 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total, Gross Unrealized Loss |  |  |  |  | 77,715 | 383 | 9,117 | 626 | 20,555 | 528 | 46,506 |  |  |  |  |  |  |  |  |  |  |
Total, Gross Unrealized Loss |  |  | $ 77,715 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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- Definition Amount of accumulated pre-tax unrealized loss on investments in available-for-sale securities that have been in a loss position for twelve months or longer impacting investments. + References No definition available. + Details
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- Definition Sum of the gross unrealized losses as of the balance sheet date on all investments. + References No definition available. + Details
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