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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 VIEs (Detail) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
Variable Interest Entities [Line Items]    
Maximum Exposure To Loss $ 97,133,176 [1] $ 111,748,270 [1]
Insurance Assets 1,968,000 [2] 2,196,161 [2]
Insurance Liabilities 8,742,567 [3] 9,207,107 [3]
Derivative Liabilities 177,386 [4] 183,666 [4]
Collateralized debt obligations [Member]
    
Variable Interest Entities [Line Items]    
Maximum Exposure To Loss 11,429,975 [1] 14,093,243 [1]
Insurance Assets 11,698 [2] 15,096 [2]
Insurance Liabilities 18,144 [3] 105,219 [3]
Derivative Liabilities 103,538 [4] 120,614 [4]
Residential Mortgage-backed securities [Member]
    
Variable Interest Entities [Line Items]    
Maximum Exposure To Loss 24,927,285 [1] 30,307,753 [1]
Insurance Assets 683,370 [2] 783,329 [2]
Insurance Liabilities 4,666,502 [3] 5,396,711 [3]
Commercial Mortgage-backed securities [Member]
    
Variable Interest Entities [Line Items]    
Maximum Exposure To Loss 634,636 [1] 685,870 [1]
Derivative Liabilities 2,451 [4] 6,241 [4]
Other consumer asset-backed [Member]
    
Variable Interest Entities [Line Items]    
Maximum Exposure To Loss 6,607,902 [1] 7,851,980 [1]
Insurance Assets 121,136 [2] 129,234 [2]
Insurance Liabilities 1,205,301 [3] 1,315,146 [3]
Derivative Liabilities 37,747 [4] 35,583 [4]
Other commercial asset-backed securities [Member]
    
Variable Interest Entities [Line Items]    
Maximum Exposure To Loss 10,676,275 [1] 13,363,434 [1]
Insurance Assets 464,962 [2] 559,884 [2]
Insurance Liabilities 1,562,914 [3] 1,045,477 [3]
Derivative Liabilities 6,945 [4] 4,541 [4]
Other [Member]
    
Variable Interest Entities [Line Items]    
Maximum Exposure To Loss 5,619,718 [1] 7,552,264 [1]
Insurance Assets 132,539 [2] 139,586 [2]
Insurance Liabilities 606,137 [3] 643,864 [3]
Derivative Liabilities 3,455 [4] 1,837 [4]
Global Structured Finance [Member]
    
Variable Interest Entities [Line Items]    
Maximum Exposure To Loss 59,895,791 [1] 73,854,544 [1]
Insurance Assets 1,413,705 [2] 1,627,129 [2]
Insurance Liabilities 8,058,998 [3] 8,506,417 [3]
Derivative Liabilities 154,136 [4] 168,816 [4]
Global Public Finance [Member]
    
Variable Interest Entities [Line Items]    
Maximum Exposure To Loss 37,237,385 [1] 37,893,726 [1]
Insurance Assets 554,295 [2] 569,032 [2]
Insurance Liabilities 683,569 [3] 700,690 [3]
Derivative Liabilities $ 23,250 [4] $ 14,850 [4]
[1] Maximum exposure to loss represents the gross maximum future payments of principal and interest on insured obligations and credit derivative contracts. Ambac's maximum exposure to loss does not include the benefit of any financial instruments (such as reinsurance or hedge contracts) that Ambac may utilize to mitigate the risks associated with these variable interests.
[2] Insurance assets represent the amount recorded in "Premium receivables" and "Subrogation recoverable" for financial guarantee contracts on Ambac's Consolidated Balance Sheets.
[3] Insurance liabilities represent the amount recorded in "Losses and loss expense reserve" and "Unearned premiums" for financial guarantee contracts on Ambac's Consolidated Balance Sheets.
[4] Derivative liabilities represent the fair value recognized on credit derivative contracts on Ambac's Consolidated Balance Sheets.