Fresh Start Financial Statement Reporting - Schedule of Calculation of Pre-Tax Gain, Recorded as Reorganization on Consolidated Statement of Total Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	Apr. 30, 2013
Extinguishment of Debt [Line Items]
Liabilities subject to compromise	$ (1,704,641)
Long-term debt	973
Accrued interest payable	821
Total debt discharged	1,706,435
Less: Successor Ambac common stock	(450)
Successor Ambac additional paid-in capital	(184,550)
Pre-tax gain from cancellation and satisfaction of Predecessor Ambac debt	1,521,435
Predecessor [Member]
Extinguishment of Debt [Line Items]
Liabilities subject to compromise	(1,704,641)
Predecessor [Member] | Common Stock Successor Ambac [Member]
Extinguishment of Debt [Line Items]
Less: Successor Ambac common stock	0
Predecessor [Member] | Additional Paid-In Capital Successor [Member]
Extinguishment of Debt [Line Items]
Successor Ambac additional paid-in capital	$ 0