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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
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