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Commitments and Contingencies - Additional Information (Detail) (USD $) | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Dec. 31, 2012 | Dec. 31, 2011 | Jun. 30, 2012 | Jun. 04, 2012 | Jun. 14, 2011 | May 31, 2011 | Dec. 31, 2010
Contingencies And Commitments [Line Items] |  |  |  |  |  |  |
Rent expense | $ 5,252,000 | $ 6,980,000 |  |  |  |  |
Total par value of repurchased surplus notes |  |  | 789,179,000 | 789,179,000 |  |  |
Principal amount of surplus notes |  |  |  | 188,446,000 |  |  |
IRS claim in the Bankruptcy Court |  |  |  |  | 807,244,000 | 807,244,000 |
Loss carry-forwards | 3,400,000 |  |  |  |  |  | 3,400,000,000
Payment by subsidiary associated with NOL Usage Tier C | 12.50% |  |  |  |  |  |
Payment by subsidiary associated with NOL Usage Tier D | 17.50% |  |  |  |  |  |
Ambac Assurance's payment under a swap termination surety bond | 4,524,000 |  |  |  |  |  |
Ambac [Member] |  |  |  |  |  |  |
Contingencies And Commitments [Line Items] |  |  |  |  |  |  |
Payment to settle the IRS dispute and related proceedings | 1,900,000 |  |  |  |  |  |
Bonds redeemed | 2,700,000 |  |  |  |  |  |
Ambac Assurance [Member] |  |  |  |  |  |  |
Contingencies And Commitments [Line Items] |  |  |  |  |  |  |
Payment to settle the IRS dispute and related proceedings | 100,000,000 |  |  |  |  |  |
Ambac Assurance's payment under a swap termination surety bond | $ 800,000,000 |  |  |  |  |  |
IRS [Member] |  |  |  |  |  |  |
Contingencies And Commitments [Line Items] |  |  |  |  |  |  |
Payment by subsidiary associated with NOL Usage Tier C | 12.50% |  |  |  |  |  |
Payment by subsidiary associated with NOL Usage Tier D | 17.50% |  |  |  |  |  |

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