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Fair Value Measurements - Summary of Changes in Level 3 Fair Value Category (Detail) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 4 Months Ended | 9 Months Ended | 3 Months Ended | 4 Months Ended | 5 Months Ended | 9 Months Ended | 3 Months Ended | 4 Months Ended | 9 Months Ended | 3 Months Ended | 4 Months Ended | 9 Months Ended | 3 Months Ended | 4 Months Ended | 9 Months Ended | 3 Months Ended | 4 Months Ended | 9 Months Ended | 3 Months Ended | 4 Months Ended | 5 Months Ended | 9 Months Ended | 3 Months Ended | 4 Months Ended | 5 Months Ended | 9 Months Ended | 3 Months Ended | 4 Months Ended | 5 Months Ended | 9 Months Ended | 3 Months Ended | 5 Months Ended | 3 Months Ended | 5 Months Ended | 3 Months Ended | 5 Months Ended | 3 Months Ended | 5 Months Ended | 3 Months Ended | 5 Months Ended | 3 Months Ended | 4 Months Ended | 9 Months Ended | 3 Months Ended | 4 Months Ended | 9 Months Ended | 3 Months Ended | 4 Months Ended | 9 Months Ended | 3 Months Ended | 4 Months Ended | 9 Months Ended | 3 Months Ended | 4 Months Ended | 9 Months Ended | 3 Months Ended | 4 Months Ended | 9 Months Ended | 3 Months Ended | 4 Months Ended | 9 Months Ended | 3 Months Ended | 5 Months Ended | 3 Months Ended | 5 Months Ended | 3 Months Ended | 5 Months Ended | 3 Months Ended | 5 Months Ended | 3 Months Ended | 5 Months Ended | 3 Months Ended | 5 Months Ended | 3 Months Ended | 5 Months Ended
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Sep. 30, 2012 Predecessor Ambac [Member] Investment by Class [Member] | Apr. 30, 2013 Predecessor Ambac [Member] Investment by Class [Member] | Sep. 30, 2012 Predecessor Ambac [Member] Investment by Class [Member] | Sep. 30, 2012 Predecessor Ambac [Member] Derivatives by Class [Member] | Apr. 30, 2013 Predecessor Ambac [Member] Derivatives by Class [Member] | Sep. 30, 2013 Predecessor Ambac [Member] Derivatives by Class [Member] | Sep. 30, 2012 Predecessor Ambac [Member] Derivatives by Class [Member] | Sep. 30, 2012 Predecessor Ambac [Member] Other Asset Backed Securities [Member] Investment by Class [Member] | Apr. 30, 2013 Predecessor Ambac [Member] Other Asset Backed Securities [Member] Investment by Class [Member] | Sep. 30, 2012 Predecessor Ambac [Member] Other Asset Backed Securities [Member] Investment by Class [Member] | Sep. 30, 2012 Predecessor Ambac [Member] U.S. Agency Obligations [Member] Investment by Class [Member] | Apr. 30, 2013 Predecessor Ambac [Member] U.S. Agency Obligations [Member] Investment by Class [Member] | Sep. 30, 2012 Predecessor Ambac [Member] U.S. Agency Obligations [Member] Investment by Class [Member] | Sep. 30, 2012 Predecessor Ambac [Member] Collateralized Debt Obligations [Member] Investment by Class [Member] | Apr. 30, 2013 Predecessor Ambac [Member] Collateralized Debt Obligations [Member] Investment by Class [Member] | Sep. 30, 2012 Predecessor Ambac [Member] Collateralized Debt Obligations [Member] Investment by Class [Member] | Sep. 30, 2012 Predecessor Ambac [Member] Corporate Obligations [Member] Investment by Class [Member] | Apr. 30, 2013 Predecessor Ambac [Member] Corporate Obligations [Member] Investment by Class [Member] | Sep. 30, 2012 Predecessor Ambac [Member] Corporate Obligations [Member] Investment by Class [Member] | Sep. 30, 2012 Predecessor Ambac [Member] Interest Rate Swaps [Member] Derivatives by Class [Member] | Apr. 30, 2013 Predecessor Ambac [Member] Interest Rate Swaps [Member] Derivatives by Class [Member] | Sep. 30, 2013 Predecessor Ambac [Member] Interest Rate Swaps [Member] Derivatives by Class [Member] | Sep. 30, 2012 Predecessor Ambac [Member] Interest Rate Swaps [Member] Derivatives by Class [Member] | Sep. 30, 2012 Predecessor Ambac [Member] Credit Derivatives [Member] Derivatives by Class [Member] | Apr. 30, 2013 Predecessor Ambac [Member] Credit Derivatives [Member] Derivatives by Class [Member] | Sep. 30, 2013 Predecessor Ambac [Member] Credit Derivatives [Member] Derivatives by Class [Member] | Sep. 30, 2012 Predecessor Ambac [Member] Credit Derivatives [Member] Derivatives by Class [Member] | Sep. 30, 2012 Predecessor Ambac [Member] Call Options on Long-Term Debt [Member] Derivatives by Class [Member] | Apr. 30, 2013 Predecessor Ambac [Member] Call Options on Long-Term Debt [Member] Derivatives by Class [Member] | Sep. 30, 2013 Predecessor Ambac [Member] Call Options on Long-Term Debt [Member] Derivatives by Class [Member] | Sep. 30, 2012 Predecessor Ambac [Member] Call Options on Long-Term Debt [Member] Derivatives by Class [Member] | Sep. 30, 2013 Successor Ambac [Member] Investment by Class [Member] | Sep. 30, 2013 Successor Ambac [Member] Investment by Class [Member] | Sep. 30, 2013 Successor Ambac [Member] Derivatives by Class [Member] | Sep. 30, 2013 Successor Ambac [Member] Other Asset Backed Securities [Member] Investment by Class [Member] | Sep. 30, 2013 Successor Ambac [Member] Other Asset Backed Securities [Member] Investment by Class [Member] | Sep. 30, 2013 Successor Ambac [Member] U.S. Agency Obligations [Member] Investment by Class [Member] | Sep. 30, 2013 Successor Ambac [Member] U.S. Agency Obligations [Member] Investment by Class [Member] | Sep. 30, 2013 Successor Ambac [Member] Collateralized Debt Obligations [Member] Investment by Class [Member] | Sep. 30, 2013 Successor Ambac [Member] Collateralized Debt Obligations [Member] Investment by Class [Member] | Sep. 30, 2013 Successor Ambac [Member] Corporate Obligations [Member] Investment by Class [Member] | Sep. 30, 2013 Successor Ambac [Member] Corporate Obligations [Member] Investment by Class [Member] | Sep. 30, 2013 Successor Ambac [Member] Interest Rate Swaps [Member] Derivatives by Class [Member] | Sep. 30, 2013 Successor Ambac [Member] Credit Derivatives [Member] Derivatives by Class [Member] | Sep. 30, 2013 Successor Ambac [Member] Call Options on Long-Term Debt [Member] Derivatives by Class [Member] | Sep. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] | Apr. 30, 2013 Level 3 [Member] Predecessor Ambac [Member] | Sep. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] | Sep. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] Investments Contract [Member] | Apr. 30, 2013 Level 3 [Member] Predecessor Ambac [Member] Investments Contract [Member] | Sep. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] Investments Contract [Member] | Sep. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] Loans [Member] | Apr. 30, 2013 Level 3 [Member] Predecessor Ambac [Member] Loans [Member] | Sep. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] Loans [Member] | Sep. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] Long-Term Debt [Member] | Apr. 30, 2013 Level 3 [Member] Predecessor Ambac [Member] Long-Term Debt [Member] | Sep. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] Long-Term Debt [Member] | Sep. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] Investments [Member] | Apr. 30, 2013 Level 3 [Member] Predecessor Ambac [Member] Investments [Member] | Sep. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] Investments [Member] | Sep. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] Other Assets [Member] | Apr. 30, 2013 Level 3 [Member] Predecessor Ambac [Member] Other Assets [Member] | Sep. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] Other Assets [Member] | Sep. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] Derivatives Assets [Member] | Apr. 30, 2013 Level 3 [Member] Predecessor Ambac [Member] Derivatives Assets [Member] | Sep. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] Derivatives Assets [Member] | Sep. 30, 2013 Level 3 [Member] Successor Ambac [Member] | Sep. 30, 2013 Level 3 [Member] Successor Ambac [Member] | Sep. 30, 2013 Level 3 [Member] Successor Ambac [Member] Investments Contract [Member] | Sep. 30, 2013 Level 3 [Member] Successor Ambac [Member] Investments Contract [Member] | Sep. 30, 2013 Level 3 [Member] Successor Ambac [Member] Loans [Member] | Sep. 30, 2013 Level 3 [Member] Successor Ambac [Member] Loans [Member] | Sep. 30, 2013 Level 3 [Member] Successor Ambac [Member] Long-Term Debt [Member] | Sep. 30, 2013 Level 3 [Member] Successor Ambac [Member] Long-Term Debt [Member] | Sep. 30, 2013 Level 3 [Member] Successor Ambac [Member] Investments [Member] | Sep. 30, 2013 Level 3 [Member] Successor Ambac [Member] Investments [Member] | Sep. 30, 2013 Level 3 [Member] Successor Ambac [Member] Other Assets [Member] | Sep. 30, 2013 Level 3 [Member] Successor Ambac [Member] Other Assets [Member] | Sep. 30, 2013 Level 3 [Member] Successor Ambac [Member] Derivatives Assets [Member] | Sep. 30, 2013 Level 3 [Member] Successor Ambac [Member] Derivatives Assets [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Balance, beginning of period | $ 63,770 | $ 60,402 | $ 97,522 | $ (507,604) | $ (322,337) | $ (415,360) | $ (486,775) | $ 50,746 | $ 50,264 | $ 75,886 |  |  | $ 1,224 | $ 9,558 | $ 6,482 | $ 12,482 | $ 3,466 | $ 3,656 | $ 7,930 | $ (295,987) | $ (108,752) | $ (137,947) | $ (302,177) | $ (211,617) | $ (213,585) | $ (277,413) | $ (190,653) |  |  |  | $ 6,055 | $ 68,572 | $ 69,412 | $ (340,663) | $ 61,736 | $ 61,782 |  |  | $ 3,415 | $ 3,949 | $ 3,421 | $ 3,681 | $ (108,396) | $ (232,267) |  | $ 13,626,233 | $ 14,416,488 | $ 14,019,216 | $ 2,162,062 | $ 2,261,294 | $ 2,199,338 | $ 14,238,273 | $ 15,359,073 | $ 14,126,994 | $ (2,346,060) | $ (2,956,501) | $ (1,934,642) | $ 63,770 | $ 60,402 | $ 97,522 | $ 15,792 | $ 14,557 | $ 16,779 | $ (507,604) | $ (322,337) | $ (486,775) | $ 14,231,973 | $ 15,170,359 | $ 2,242,989 | $ 2,500,565 | $ 13,820,878 | $ 14,752,053 | $ (1,573,687) | $ (1,750,372) | $ 68,572 | $ 69,412 | $ 13,884 | $ 14,061 | $ (340,663) | $ (415,360)
Additions of VIEs consolidated |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (409,300) |  |  |  |  |  |  |  |  | (409,300) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Included in earnings | (22) | (33) | 17 | 26,285 | (88,546) | 107,253 | 67,322 |  |  |  |  |  | (2) | (2) | (6) | (5) | (20) | (27) | 24 | (1,156) | (28,162) | 24,838 | (46,191) | 27,441 | (60,384) | 82,415 | 12,803 |  |  |  | 100,710 | 467 | 863 | 37,126 | 483 | 876 |  |  | 7 | 25 | (23) | (38) | 5,931 | 31,195 |  | 367,070 | 1,057,181 | 527,590 | (64,826) | 328,768 | (127,539) | 439,649 | 956,402 | 738,121 | (33,245) | (138,914) | (148,573) | (22) | (33) | 17 | (771) | (496) | (1,758) | 26,285 | (88,546) | 67,322 | 291,195 | (429,751) | 36,694 | (172,304) | 195,305 | (391,336) | 21,816 | 26,163 | 467 | 863 | (213) | (390) | 37,126 | 107,253
Included in other comprehensive income | 471 | 12,329 | 6,697 |  |  |  |  | 122 | 12,117 | 6,498 |  |  | (9) | 200 | 160 | 492 | 149 | 52 | (284) |  |  |  |  |  |  |  |  |  |  |  |  | 5,951 | 6,013 |  | 5,923 | 6,242 |  |  | (7) | (19) | 35 | (210) |  |  |  | 386,830 | (776,014) | 558,465 | 62,877 | (89,497) | 88,314 | 392,548 | (849,833) | 541,215 | (69,066) | 150,987 | (77,761) | 471 | 12,329 | 6,697 |  |  |  |  |  |  | 916,364 | 613,641 | 144,231 | 95,653 | 842,715 | 560,294 | (76,533) | (48,319) | 5,951 | 6,013 |  |  |  |
Purchases |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuances |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Sales |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Settlements | (3,497) | (3,286) | (96,118) | 7,278 | (4,477) | 2,433 | (54,588) | (573) | (599) | (85,157) |  |  | (749) | (2,924) | (2,687) | (6,137) |  |  | (4,075) | 10,223 | (1,033) | 10,088 | 61,448 | (2,945) | (3,444) | (7,655) | (9,271) |  |  |  | (106,765) | (4,192) | (5,490) | (2,137) | (777) | (1,535) |  |  | (3,415) | (3,955) |  |  | (556) | (1,581) |  | (50,728) | (716,488) | (508,182) |  |  |  | (85,811) | (713,589) | (421,671) | 31,302 | 4,864 | 64,195 | (3,497) | (3,286) | (96,118) |  |  |  | 7,278 | (4,477) | (54,588) | (98,528) | (157,369) |  |  | (96,452) | (158,565) | 4,253 | 4,253 | (4,192) | (5,490) |  |  | (2,137) | 2,433
Transfers in Level 3 |  |  | 58,905 |  |  |  |  |  |  | 53,068 |  |  |  |  |  |  |  |  | 5,837 |  |  |  |  |  |  |  |  |  |  |  |  | 273 | 273 |  |  |  | 273 | 273 |  |  |  |  |  |  |  | (576,272) |  | (1,494,714) |  |  |  |  |  |  | (576,272) |  | (1,553,619) |  |  | 58,905 |  |  |  |  |  |  | 273 | (220,649) |  |  |  |  |  | (220,922) | 273 | 273 |  |  |  |
Transfers out of Level 3 |  |  | (6,301) |  |  |  |  |  |  |  |  |  | (464) |  |  |  |  |  | (5,837) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,598,492 | 650,758 |  |  |  |  |  |  |  | 1,598,492 | 657,059 |  |  | (6,301) |  |  |  |  |  |  |  | 365,046 |  |  |  |  |  | 365,046 |  |  |  |  |  |
Deconsolidation of VIEs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Balance, end of period | 60,722 | 69,412 | 60,722 | (474,041) | (415,360) | (305,674) | (474,041) | 50,295 | 61,782 | 50,295 |  |  |  | 6,832 | 3,949 | 6,832 | 3,595 | 3,681 | 3,595 | (286,920) | (137,947) | (103,021) | (286,920) | (187,121) | (277,413) | (202,653) | (187,121) |  |  |  |  | 71,071 | 71,071 | (305,674) | 67,365 | 67,365 | 273 | 273 |  |  | 3,433 | 3,433 | (103,021) | (202,653) |  | 13,753,133 | 15,170,359 | 13,753,133 | 2,160,113 | 2,500,565 | 2,160,113 | 14,984,659 | 14,752,053 | 14,984,659 | (2,993,341) | (1,750,372) | (2,993,341) | 60,722 | 69,412 | 60,722 | 15,021 | 14,061 | 15,021 | (474,041) | (415,360) | (474,041) | 15,341,277 | 15,341,277 | 2,423,914 | 2,423,914 | 14,762,446 | 14,762,446 | (1,624,151) | (1,624,151) | 71,071 | 71,071 | 13,671 | 13,671 | (305,674) | (305,674)
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date |  |  |  | $ 22,792 | $ (92,002) | $ 65,509 | $ (51,611) |  |  |  |  |  |  |  |  |  |  |  |  | $ (1,156) | $ (28,162) | $ 24,838 | $ (47,419) | $ 23,948 | $ (63,840) | $ 40,671 | $ (4,192) |  |  |  |  |  |  | $ 32,984 |  |  |  |  |  |  |  |  | $ 5,931 | $ 27,053 |  | $ 363,599 | $ 1,053,758 | $ 409,121 | $ (64,826) | $ 328,768 | $ (127,539) | $ 439,649 | $ 956,402 | $ 738,602 | $ (33,245) | $ (138,914) | $ (148,573) |  |  |  | $ (771) | $ (496) | $ (1,758) | $ 22,792 | $ (92,002) | $ (51,611) | $ 286,586 | $ (472,358) | $ 36,694 | $ (172,304) | $ 195,305 | $ (391,336) | $ 21,816 | $ 26,163 |  |  | $ (213) | $ (390) | $ 32,984 | $ 65,509

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