Comprehensive Income - Schedule of Changes in Balances of Each Component of Accumulated Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	5 Months Ended	4 Months Ended	3 Months Ended	5 Months Ended	4 Months Ended	3 Months Ended	5 Months Ended	4 Months Ended
Sep. 30, 2013 Successor Ambac [Member]	Sep. 30, 2013 Successor Ambac [Member]	Apr. 30, 2013 Predecessor Ambac [Member]	Sep. 30, 2013 Unrealized Gains (Losses) on Available-for-Sale Securities [Member] Successor Ambac [Member]	Sep. 30, 2013 Unrealized Gains (Losses) on Available-for-Sale Securities [Member] Successor Ambac [Member]	Apr. 30, 2013 Unrealized Gains (Losses) on Available-for-Sale Securities [Member] Predecessor Ambac [Member]	Apr. 30, 2013 Amortization of Postretirement Benefit [Member] Predecessor Ambac [Member]	Sep. 30, 2013 Gain (Loss) on Foreign Currency Translation [Member] Successor Ambac [Member]	Sep. 30, 2013 Gain (Loss) on Foreign Currency Translation [Member] Successor Ambac [Member]	Apr. 30, 2013 Gain (Loss) on Foreign Currency Translation [Member] Predecessor Ambac [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ (103,507)		$ 625,385	$ (90,983)		$ 651,272	$ (5,860)	$ (12,524)		$ (20,027)
Other comprehensive income before reclassifications	43,700	(43,335)	187,555	4,316	(70,195)	188,212		39,384	26,860	(657)
Amounts reclassified from accumulated other comprehensive income	49,561	33,089	(12,680)	49,561	33,089	(12,865)	185
Elimination of Predecessor Ambac Shareholder Equity Accounts			(800,260)			(826,619)	5,675			20,684
Net current period other comprehensive income	93,261	(10,246)	(625,385)	53,877	(37,106)	(651,272)	5,860	39,384	26,860	20,027
Ending Balance	$ (10,246)	$ (10,246)	$ 800,260	$ (37,106)	$ (37,106)			$ 26,860	$ 26,860

- Definition

Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

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