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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
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