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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 | Mar. 31, 2013 | Dec. 31, 2012
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Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | $ 221,293 | $ 227,248
Less than 12 Months, Gross Unrealized Loss | 5,156 | 7,551
12 Months or More, Fair Value | 355,075 | 455,626
12 Months or More, Gross Unrealized Loss | 61,709 | 70,164
Total, Fair Value | 576,368 | 682,874
Total, Gross Unrealized Loss | 66,865 | 77,715
Less than 12 Months, Fair Value | 223,296 | 228,442
Less than 12 Months, Gross Unrealized Loss | 5,156 | 7,551
12 Months or More, Fair Value | 355,075 | 455,626
12 Months or More, Gross Unrealized Losses | 61,709 | 70,164
Total, Fair Value | 578,371 | 684,068
Gross Unrealized Loss On available-for-sale Investment | 66,865 | 77,715
Short-term [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 2,003 | 1,194
Total, Fair Value | 2,003 | 1,194
Municipal obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 42,336 | 42,503
Less than 12 Months, Gross Unrealized Loss | 627 | 354
12 Months or More, Fair Value | 3,958 | 4,303
12 Months or More, Gross Unrealized Loss | 15 | 29
Total, Fair Value | 46,294 | 46,806
Total, Gross Unrealized Loss | 642 | 383
Corporate obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 16,506 | 69,727
Less than 12 Months, Gross Unrealized Loss | 156 | 1,081
12 Months or More, Fair Value | 85,494 | 132,916
12 Months or More, Gross Unrealized Loss | 6,868 | 8,036
Total, Fair Value | 102,000 | 202,643
Total, Gross Unrealized Loss | 7,024 | 9,117
U.S. government obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value |  | 26,081
Less than 12 Months, Gross Unrealized Loss |  | 626
Total, Fair Value |  | 26,081
Total, Gross Unrealized Loss |  | 626
Residential mortgage-backed securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 43,511 | 88,504
Less than 12 Months, Gross Unrealized Loss | 4,071 | 5,319
12 Months or More, Fair Value | 111,909 | 116,146
12 Months or More, Gross Unrealized Loss | 10,328 | 15,236
Total, Fair Value | 155,420 | 204,650
Total, Gross Unrealized Loss | 14,399 | 20,555
Collateralized debt obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 860 | 253
Less than 12 Months, Gross Unrealized Loss | 152 | 168
12 Months or More, Fair Value | 3,905 | 13,429
12 Months or More, Gross Unrealized Loss | 72 | 360
Total, Fair Value | 4,765 | 13,682
Total, Gross Unrealized Loss | 224 | 528
Other asset-backed securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 118,080 | 180
Less than 12 Months, Gross Unrealized Loss | 150 | 3
12 Months or More, Fair Value | 149,809 | 188,832
12 Months or More, Gross Unrealized Loss | 44,426 | 46,503
Total, Fair Value | 267,889 | 189,012
Total, Gross Unrealized Loss | $ 44,576 | $ 46,506

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