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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
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