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Special Purpose Entities, Including Variable Interest Entities - Summary of Carrying Amount of Assets, Liabilities and Maximum Exposure to Loss of Ambac's Variable Interests in Non-Consolidated Variable Interest Entities (Detail) (Variable interest entity [Member], USD $)
In Thousands, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Variable Interest Entities [Line Items]    
Maximum Exposure To Loss $ 87,781,977 $ 93,517,995
Insurance Assets 1,825,890 1,840,265
Insurance Liabilities 7,417,924 7,463,635
Derivative Liabilities 189,722 201,434
Global Structured Finance [Member]
    
Variable Interest Entities [Line Items]    
Maximum Exposure To Loss 52,051,079 56,421,767
Insurance Assets 1,301,581 1,298,086
Insurance Liabilities 6,810,943 6,830,277
Derivative Liabilities 160,252 172,771
Global Public Finance [Member]
    
Variable Interest Entities [Line Items]    
Maximum Exposure To Loss 35,730,898 37,096,228
Insurance Assets 524,309 542,179
Insurance Liabilities 606,981 633,358
Derivative Liabilities 29,470 28,663
Collateralized debt obligations [Member] | Global Structured Finance [Member]
    
Variable Interest Entities [Line Items]    
Maximum Exposure To Loss 8,880,445 10,176,522
Insurance Assets 7,656 9,673
Insurance Liabilities 10,922 13,328
Derivative Liabilities 104,544 113,057
Residential mortgage-backed securities [Member] | Global Structured Finance [Member]
    
Variable Interest Entities [Line Items]    
Maximum Exposure To Loss 21,936,226 24,008,616
Insurance Assets 645,742 603,867
Insurance Liabilities 3,875,478 3,969,336
Mortgage-backed-commercial [Member] | Global Structured Finance [Member]
    
Variable Interest Entities [Line Items]    
Maximum Exposure To Loss 610,872 643,387
Derivative Liabilities 2,788 2,418
Other consumer asset-backed [Member] | Global Structured Finance [Member]
    
Variable Interest Entities [Line Items]    
Maximum Exposure To Loss 5,755,161 5,895,377
Insurance Assets 98,185 101,494
Insurance Liabilities 1,120,657 1,042,522
Derivative Liabilities 38,018 45,610
Other commercial asset-backed [Member] | Global Structured Finance [Member]
    
Variable Interest Entities [Line Items]    
Maximum Exposure To Loss 9,611,024 10,192,858
Insurance Assets 420,513 451,048
Insurance Liabilities 1,196,286 1,215,074
Derivative Liabilities 9,427 7,293
Other [Member] | Global Structured Finance [Member]
    
Variable Interest Entities [Line Items]    
Maximum Exposure To Loss 5,257,351 5,505,007
Insurance Assets 129,485 132,004
Insurance Liabilities 607,600 590,017
Derivative Liabilities $ 5,475 $ 4,393