Segment Information - Summary of Financial Information by Reportable Segment (Detail) (USD $) In Thousands, unless otherwise specified		3 Months Ended	5 Months Ended		3 Months Ended	5 Months Ended	3 Months Ended	4 Months Ended	9 Months Ended		3 Months Ended	4 Months Ended	9 Months Ended	3 Months Ended	5 Months Ended	3 Months Ended	5 Months Ended	3 Months Ended	5 Months Ended	3 Months Ended	5 Months Ended	3 Months Ended	5 Months Ended	3 Months Ended	5 Months Ended	3 Months Ended	4 Months Ended	9 Months Ended	3 Months Ended	4 Months Ended	9 Months Ended	3 Months Ended	4 Months Ended	9 Months Ended	3 Months Ended	4 Months Ended	9 Months Ended	3 Months Ended	4 Months Ended	9 Months Ended	3 Months Ended	4 Months Ended	9 Months Ended	3 Months Ended	5 Months Ended	3 Months Ended	5 Months Ended	3 Months Ended	5 Months Ended	3 Months Ended	4 Months Ended	9 Months Ended	3 Months Ended	4 Months Ended	9 Months Ended	3 Months Ended	4 Months Ended	9 Months Ended	3 Months Ended	5 Months Ended	3 Months Ended	5 Months Ended	3 Months Ended	4 Months Ended	9 Months Ended	3 Months Ended	4 Months Ended	9 Months Ended
	Apr. 30, 2013	Sep. 30, 2013 Successor Ambac [Member]	Sep. 30, 2013 Successor Ambac [Member]	Apr. 30, 2013 Successor Ambac [Member]	Sep. 30, 2013 Successor Ambac [Member] Unaffiliated Customers [Member]	Sep. 30, 2013 Successor Ambac [Member] Unaffiliated Customers [Member]	Sep. 30, 2012 Predecessor Ambac [Member]	Apr. 30, 2013 Predecessor Ambac [Member]	Sep. 30, 2012 Predecessor Ambac [Member]	Dec. 31, 2012 Predecessor Ambac [Member]	Sep. 30, 2012 Predecessor Ambac [Member] Unaffiliated Customers [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Unaffiliated Customers [Member]	Sep. 30, 2012 Predecessor Ambac [Member] Unaffiliated Customers [Member]	Sep. 30, 2013 Operating Segments [Member] Successor Ambac [Member] Financial Guarantee [Member]	Sep. 30, 2013 Operating Segments [Member] Successor Ambac [Member] Financial Guarantee [Member]	Sep. 30, 2013 Operating Segments [Member] Successor Ambac [Member] Financial Guarantee [Member] Unaffiliated Customers [Member]	Sep. 30, 2013 Operating Segments [Member] Successor Ambac [Member] Financial Guarantee [Member] Unaffiliated Customers [Member]	Sep. 30, 2013 Operating Segments [Member] Successor Ambac [Member] Financial Guarantee [Member] Inter-Segment [Member]	Sep. 30, 2013 Operating Segments [Member] Successor Ambac [Member] Financial Guarantee [Member] Inter-Segment [Member]	Sep. 30, 2013 Operating Segments [Member] Successor Ambac [Member] Financial Services [Member]	Sep. 30, 2013 Operating Segments [Member] Successor Ambac [Member] Financial Services [Member]	Sep. 30, 2013 Operating Segments [Member] Successor Ambac [Member] Financial Services [Member] Unaffiliated Customers [Member]	Sep. 30, 2013 Operating Segments [Member] Successor Ambac [Member] Financial Services [Member] Unaffiliated Customers [Member]	Sep. 30, 2013 Operating Segments [Member] Successor Ambac [Member] Financial Services [Member] Inter-Segment [Member]	Sep. 30, 2013 Operating Segments [Member] Successor Ambac [Member] Financial Services [Member] Inter-Segment [Member]	Sep. 30, 2012 Operating Segments [Member] Predecessor Ambac [Member] Financial Guarantee [Member]	Apr. 30, 2013 Operating Segments [Member] Predecessor Ambac [Member] Financial Guarantee [Member]	Sep. 30, 2012 Operating Segments [Member] Predecessor Ambac [Member] Financial Guarantee [Member]	Sep. 30, 2012 Operating Segments [Member] Predecessor Ambac [Member] Financial Guarantee [Member] Unaffiliated Customers [Member]	Apr. 30, 2013 Operating Segments [Member] Predecessor Ambac [Member] Financial Guarantee [Member] Unaffiliated Customers [Member]	Sep. 30, 2012 Operating Segments [Member] Predecessor Ambac [Member] Financial Guarantee [Member] Unaffiliated Customers [Member]	Sep. 30, 2012 Operating Segments [Member] Predecessor Ambac [Member] Financial Guarantee [Member] Inter-Segment [Member]	Apr. 30, 2013 Operating Segments [Member] Predecessor Ambac [Member] Financial Guarantee [Member] Inter-Segment [Member]	Sep. 30, 2012 Operating Segments [Member] Predecessor Ambac [Member] Financial Guarantee [Member] Inter-Segment [Member]	Sep. 30, 2012 Operating Segments [Member] Predecessor Ambac [Member] Financial Services [Member]	Apr. 30, 2013 Operating Segments [Member] Predecessor Ambac [Member] Financial Services [Member]	Sep. 30, 2012 Operating Segments [Member] Predecessor Ambac [Member] Financial Services [Member]	Sep. 30, 2012 Operating Segments [Member] Predecessor Ambac [Member] Financial Services [Member] Unaffiliated Customers [Member]	Apr. 30, 2013 Operating Segments [Member] Predecessor Ambac [Member] Financial Services [Member] Unaffiliated Customers [Member]	Sep. 30, 2012 Operating Segments [Member] Predecessor Ambac [Member] Financial Services [Member] Unaffiliated Customers [Member]	Sep. 30, 2012 Operating Segments [Member] Predecessor Ambac [Member] Financial Services [Member] Inter-Segment [Member]	Apr. 30, 2013 Operating Segments [Member] Predecessor Ambac [Member] Financial Services [Member] Inter-Segment [Member]	Sep. 30, 2012 Operating Segments [Member] Predecessor Ambac [Member] Financial Services [Member] Inter-Segment [Member]	Sep. 30, 2013 Corporate and Other [Member] Successor Ambac [Member]	Sep. 30, 2013 Corporate and Other [Member] Successor Ambac [Member]	Sep. 30, 2013 Corporate and Other [Member] Successor Ambac [Member] Unaffiliated Customers [Member]	Sep. 30, 2013 Corporate and Other [Member] Successor Ambac [Member] Unaffiliated Customers [Member]	Sep. 30, 2013 Corporate and Other [Member] Successor Ambac [Member] Inter-Segment [Member]	Sep. 30, 2013 Corporate and Other [Member] Successor Ambac [Member] Inter-Segment [Member]	Sep. 30, 2012 Corporate and Other [Member] Predecessor Ambac [Member]	Apr. 30, 2013 Corporate and Other [Member] Predecessor Ambac [Member]	Sep. 30, 2012 Corporate and Other [Member] Predecessor Ambac [Member]	Sep. 30, 2012 Corporate and Other [Member] Predecessor Ambac [Member] Unaffiliated Customers [Member]	Apr. 30, 2013 Corporate and Other [Member] Predecessor Ambac [Member] Unaffiliated Customers [Member]	Sep. 30, 2012 Corporate and Other [Member] Predecessor Ambac [Member] Unaffiliated Customers [Member]	Sep. 30, 2012 Corporate and Other [Member] Predecessor Ambac [Member] Inter-Segment [Member]	Apr. 30, 2013 Corporate and Other [Member] Predecessor Ambac [Member] Inter-Segment [Member]	Sep. 30, 2012 Corporate and Other [Member] Predecessor Ambac [Member] Inter-Segment [Member]	Sep. 30, 2013 Inter-segment Eliminations [Member] Successor Ambac [Member]	Sep. 30, 2013 Inter-segment Eliminations [Member] Successor Ambac [Member]	Sep. 30, 2013 Inter-segment Eliminations [Member] Successor Ambac [Member] Inter-Segment [Member]	Sep. 30, 2013 Inter-segment Eliminations [Member] Successor Ambac [Member] Inter-Segment [Member]	Sep. 30, 2012 Inter-segment Eliminations [Member] Predecessor Ambac [Member]	Apr. 30, 2013 Inter-segment Eliminations [Member] Predecessor Ambac [Member]	Sep. 30, 2012 Inter-segment Eliminations [Member] Predecessor Ambac [Member]	Sep. 30, 2012 Inter-segment Eliminations [Member] Predecessor Ambac [Member] Inter-Segment [Member]	Apr. 30, 2013 Inter-segment Eliminations [Member] Predecessor Ambac [Member] Inter-Segment [Member]	Sep. 30, 2012 Inter-segment Eliminations [Member] Predecessor Ambac [Member] Inter-Segment [Member]
Revenues:
Total revenues before expenses and reorganization items		$ 171,652	$ 414,067		$ 171,652	$ 414,067	$ 197,162	$ 640,388	$ 515,337		$ 197,162	$ 640,388	$ 515,337	$ 157,894	$ 318,198	$ 157,382	$ 317,287	$ 512	$ 911	$ 13,672	$ 95,788	$ 14,144	$ 96,634	$ (472)	$ (846)	$ 231,258	$ 633,950	$ 591,646	$ 230,014	$ 633,010	$ 587,506	$ 1,244	$ 940	$ 4,140	$ (34,218)	$ 6,457	$ (76,437)	$ (33,021)	$ 7,339	$ (72,451)	$ (1,197)	$ (882)	$ (3,986)	$ 8,385	$ 13,830	$ 126	$ 146	$ 8,259	$ 13,684	$ 169	$ 197,094	$ 927	$ 169	$ 39	$ 282		$ 197,055	$ 645	$ (8,299)	$ (13,749)	$ (8,299)	$ (13,749)	$ (47)	$ (197,113)	$ (799)	$ (47)	$ (197,113)	$ (799)
Pre-tax income (loss) from continuing operations:
Pre-tax income (loss) from continuing operations		231,601	437,396		231,601	437,396	158,040	3,348,033	(401,899)		158,040	3,348,033	(401,899)	210,342	329,735	219,006	343,896	(8,664)	(14,161)	12,189	93,358	12,812	94,504	(623)	(1,146)	198,832	1,632,978	(311,753)	196,302	1,830,165	(314,598)	2,530	(197,187)	2,845	(37,331)	2,132	(83,702)	(35,434)	3,233	(80,021)	(1,897)	(1,101)	(3,681)	9,070	14,303	(217)	(1,004)	9,287	15,307	(3,461)	1,712,923	(6,444)	(2,828)	1,514,635	(7,280)	(633)	198,288	836
Total assets	28,853,435	28,633,638	28,633,638	28,853,435			26,947,892	28,853,435	26,947,892	27,085,265				28,117,113	28,117,113					477,111	477,111					26,206,822	28,287,321	26,206,822							703,945	536,711	703,945							39,414	39,414					37,125	29,403	37,125
Net investment income		52,126	78,322				84,078	116,740	290,031					51,637	77,652					474	632					81,037	115,129	276,251							2,986	1,572	13,611							15	38					55	39	169
Insurance intangible amortization		37,473	62,425											37,473	62,425
Interest expense		31,817	52,961				23,268	31,025	88,962					31,281	52,057					536	904					21,712	29,718	83,966							1,556	1,307	4,996
Reorganization items		$ 4	$ 428				$ 1,252	$ (2,745,180)	$ 4,480																		$ (1,231,550)									$ 1,505								$ 4	$ 428					$ 1,252	$ (1,515,135)	$ 4,480