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Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Investments (Detail) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2012 | Dec. 31, 2011
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Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | $ 227,248 | $ 635,964
Less than 12 Months, Gross Unrealized Loss | 7,551 | 30,309
12 Months or More, Fair Value | 455,626 | 516,538
12 Months or More, Gross Unrealized Loss | 70,164 | 114,011
Total, Fair Value | 682,874 | 1,152,502
Total, Gross Unrealized Loss | 77,715 | 144,320
Less than 12 Months, Fair Value | 228,242 | 641,737
Less than 12 Months, Gross Unrealized Loss | 7,551 | 30,310
12 Months or More, Fair Value | 455,626 | 516,538
12 Months or More, Gross Unrealized Losses | 70,164 | 114,011
Total, Fair Value | 684,068 | 1,158,275
Gross Unrealized Loss On Investment | 77,715 | 144,321
Short-term [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 1,194 | 5,773
Less than 12 Months, Gross Unrealized Loss |  | 1
Total, Fair Value | 1,194 | 5,773
Gross Unrealized Loss On Investment |  | 1
Municipal obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 42,503 | 9,359
Less than 12 Months, Gross Unrealized Loss | 354 | 309
12 Months or More, Fair Value | 4,303 | 14,635
12 Months or More, Gross Unrealized Loss | 29 | 174
Total, Fair Value | 46,806 | 23,994
Total, Gross Unrealized Loss | 383 | 483
Corporate obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 69,727 | 155,528
Less than 12 Months, Gross Unrealized Loss | 1,081 | 6,220
12 Months or More, Fair Value | 132,916 | 178,861
12 Months or More, Gross Unrealized Loss | 8,036 | 16,983
Total, Fair Value | 202,643 | 334,389
Total, Gross Unrealized Loss | 9,117 | 23,203
U.S. government obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 26,081 | 130,422
Less than 12 Months, Gross Unrealized Loss | 626 | 126
Total, Fair Value | 26,081 | 130,422
Total, Gross Unrealized Loss | 626 | 126
Residential mortgage-backed securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 88,504 | 125,826
Less than 12 Months, Gross Unrealized Loss | 5,319 | 14,495
12 Months or More, Fair Value | 116,146 | 105,705
12 Months or More, Gross Unrealized Loss | 15,236 | 43,321
Total, Fair Value | 204,650 | 231,531
Total, Gross Unrealized Loss | 20,555 | 57,816
Collateralized debt obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 253 | 33,037
Less than 12 Months, Gross Unrealized Loss | 168 | 840
12 Months or More, Fair Value | 13,429 | 12,482
12 Months or More, Gross Unrealized Loss | 360 | 1,751
Total, Fair Value | 13,682 | 45,519
Total, Gross Unrealized Loss | 528 | 2,591
Other asset-backed securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 180 | 181,792
Less than 12 Months, Gross Unrealized Loss | 3 | 8,319
12 Months or More, Fair Value | 188,832 | 204,855
12 Months or More, Gross Unrealized Loss | 46,503 | 51,782
Total, Fair Value | 189,012 | 386,647
Total, Gross Unrealized Loss | $ 46,506 | $ 60,101

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