Fresh Start Financial Statement Reporting - Schedule of Calculation of Pre-Tax Gain, Recorded as Reorganization on Consolidated Statement of Total Comprehensive Income (Detail) (USD $)	Apr. 30, 2013	Dec. 31, 2012
Extinguishment of Debt [Line Items]
Pre-tax gain from cancellation and satisfaction of Predecessor Ambac debt	$ 1,521,435,000
Reorganization Items Adjustments [Member]
Extinguishment of Debt [Line Items]
Liabilities subject to compromise	1,704,641,000
Long-term debt	973
Accrued interest payable	821
Total debt discharged	1,706,435,000
Reorganization Items Adjustments [Member] | Common Stock Successor Ambac [Member]
Extinguishment of Debt [Line Items]
Less: common stock	(450,000)
Reorganization Items Adjustments [Member] | Additional Paid-In Capital Successor [Member]
Extinguishment of Debt [Line Items]
Additional paid-in capital	(184,550,000)
Predecessor Ambac [Member]
Extinguishment of Debt [Line Items]
Liabilities subject to compromise	(1,704,641,000)	(1,704,904,000)
Less: common stock		(3,080,000)
Additional paid-in capital		(2,172,027,000)
Pre-tax gain from cancellation and satisfaction of Predecessor Ambac debt	1,521,435,000
Predecessor Ambac [Member] | Common Stock Successor Ambac [Member]
Extinguishment of Debt [Line Items]
Less: common stock
Predecessor Ambac [Member] | Additional Paid-In Capital Successor [Member]
Extinguishment of Debt [Line Items]
Additional paid-in capital