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Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments (Detail) (USD $) In Thousands, unless otherwise specified | Sep. 30, 2013 Successor Ambac [Member] | Sep. 30, 2013 Successor Ambac [Member] Corporate Obligations [Member] | Sep. 30, 2013 Successor Ambac [Member] Residential Mortgage-Backed Securities [Member] | Sep. 30, 2013 Successor Ambac [Member] Collateralized Debt Obligations [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] Securities Pledged as Collateral [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] Foreign 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] U.S. Government Obligations [Member] Securities Pledged as Collateral [Member] | Sep. 30, 2013 Successor Ambac [Member] Fixed Income Securities [Member] U.S. Agency 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] Asset-Backed Securities [Member] | Sep. 30, 2013 Successor Ambac [Member] Fixed Income Investments And Other Investments [Member] | Dec. 31, 2012 Predecessor Ambac [Member] | Dec. 31, 2012 Predecessor Ambac [Member] Corporate Obligations [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] Securities Pledged as Collateral [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] Foreign 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] U.S. Government Obligations [Member] Securities Pledged as Collateral [Member] | Dec. 31, 2012 Predecessor Ambac [Member] Fixed Income Securities [Member] U.S. Agency 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] Asset-Backed Securities [Member] | Dec. 31, 2012 Predecessor Ambac [Member] Fixed Income Investments And Other Investments [Member]
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Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amortized Cost | $ 6,485,273 |  | $ 1,523,755 | $ 91,023 | $ 549,233 | $ 5,795,824 | $ 140,216 | $ 1,470,363 | $ 1,367,029 | $ 127,649 | $ 106,605 | $ 140,216 | $ 35,562 | $ 1,523,755 | $ 91,023 | $ 1,073,838 | $ 6,345,057 | $ 5,678,560 |  | $ 661,219 | $ 4,751,824 | $ 265,517 | $ 1,662,124 | $ 999,554 | $ 67,347 | $ 127,037 | $ 265,517 | $ 79,295 | $ 1,096,202 | $ 32,855 | $ 687,410 | $ 5,413,043
Gross Unrealized Gains | 64,647 |  |  |  | 1 | 64,589 | 57 | 152 | 1,981 | 56 | 36 | 57 | 4 | 52,896 | 256 | 9,208 | 64,590 | 728,987 |  | 439 | 728,286 | 262 | 187,191 | 87,535 | 2,765 | 872 | 262 | 3,240 | 379,935 | 1,015 | 65,733 | 728,725
Gross Unrealized Losses | 101,753 |  |  |  | 5 | 101,748 |  | 26,157 | 24,324 | 5,206 | 1,381 |  | 77 | 21,039 | 224 | 23,340 | 101,753 | 77,715 |  |  | 77,715 |  | 383 | 9,117 |  | 626 |  |  | 20,555 | 528 | 46,506 | 77,715
Estimated Fair Value | 6,448,167 | 2,423,914 | 1,555,612 | 91,055 | 549,229 | 5,758,665 | 140,273 | 1,444,358 | 1,344,686 | 122,499 | 105,260 | 140,273 | 35,489 | 1,555,612 | 91,055 | 1,059,706 | 6,307,894 | 6,329,832 | 2,261,294 | 661,658 | 5,402,395 | 265,779 | 1,848,932 | 1,077,972 | 70,112 | 127,283 | 265,779 | 82,535 | 1,455,582 | 33,342 | 706,637 | 6,064,053
Non - credit other - than - temporary Impairments | $ 947 |  |  |  |  | $ 947 |  |  |  |  |  |  |  | $ 947 |  |  | $ 947 | $ 6,892 |  |  | $ 6,892 |  |  |  |  |  |  |  | $ 6,892 |  |  | $ 6,892

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- Definition Amount of accumulated pre-tax unrealized gain before deducting pre-tax unrealized losses on investments in available-for-sale securities impacting investments. + References No definition available. + Details
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