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Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Investments (Detail) (USD $) In Thousands, unless otherwise specified | Sep. 30, 2012 | Dec. 31, 2011
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Schedule of Available-for-sale Securities [Line Items] |  |
Less than Twelve Months, Fair Value | $ 271,941 | $ 635,964
Less than 12 Months, Gross Unrealized Loss | 14,572 | 30,309
Twelve Months or Longer, Fair Value | 522,933 | 516,538
12 Months or Longer, Gross Unrealized Loss | 80,767 | 114,011
Total, Fair Value | 794,874 | 1,152,502
Total, Gross Unrealized Loss | 95,339 | 144,320
Less than Twelve Months, Fair Value | 274,464 | 641,737
Less than 12 Months, Gross Unrealized Loss | 14,572 | 30,310
Twelve Months or Longer, Fair Value | 522,933 | 516,538
12 Months or Longer, Gross Unrealized Losses | 80,767 | 114,011
Total, Fair Value | 797,397 | 1,158,275
Total, Gross Unrealized Loss | 95,339 | 144,321
Short-term Investments [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than Twelve Months, Fair Value | 2,523 | 5,773
Less than 12 Months, Gross Unrealized Loss |  | 1
Total, Fair Value | 2,523 | 5,773
Total, Gross Unrealized Loss |  | 1
Municipal Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than Twelve Months, Fair Value | 27,609 | 9,359
Less than 12 Months, Gross Unrealized Loss | 65 | 309
Twelve Months or Longer, Fair Value | 4,294 | 14,635
12 Months or Longer, Gross Unrealized Loss | 37 | 174
Total, Fair Value | 31,903 | 23,994
Total, Gross Unrealized Loss | 102 | 483
Corporate Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than Twelve Months, Fair Value | 64,316 | 155,528
Less than 12 Months, Gross Unrealized Loss | 1,329 | 6,220
Twelve Months or Longer, Fair Value | 135,993 | 178,861
12 Months or Longer, Gross Unrealized Loss | 8,541 | 16,983
Total, Fair Value | 200,309 | 334,389
Total, Gross Unrealized Loss | 9,870 | 23,203
U.S. Government Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than Twelve Months, Fair Value | 26,099 | 130,422
Less than 12 Months, Gross Unrealized Loss | 430 | 126
Twelve Months or Longer, Fair Value | 64,970 |
12 Months or Longer, Gross Unrealized Loss | 5 |
Total, Fair Value | 91,069 | 130,422
Total, Gross Unrealized Loss | 435 | 126
Residential Mortgage-backed securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than Twelve Months, Fair Value | 143,641 | 125,826
Less than 12 Months, Gross Unrealized Loss | 12,428 | 14,495
Twelve Months or Longer, Fair Value | 112,854 | 105,705
12 Months or Longer, Gross Unrealized Loss | 23,878 | 43,321
Total, Fair Value | 256,495 | 231,531
Total, Gross Unrealized Loss | 36,306 | 57,816
Collateralized debt obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than Twelve Months, Fair Value | 260 | 33,037
Less than 12 Months, Gross Unrealized Loss | 317 | 840
Twelve Months or Longer, Fair Value | 13,780 | 12,482
12 Months or Longer, Gross Unrealized Loss | 561 | 1,751
Total, Fair Value | 14,040 | 45,519
Total, Gross Unrealized Loss | 878 | 2,591
Other commercial asset-backed securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than Twelve Months, Fair Value | 10,016 | 181,792
Less than 12 Months, Gross Unrealized Loss | 3 | 8,319
Twelve Months or Longer, Fair Value | 191,042 | 204,855
12 Months or Longer, Gross Unrealized Loss | 47,745 | 51,782
Total, Fair Value | 201,058 | 386,647
Total, Gross Unrealized Loss | $ 47,748 | $ 60,101

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