Fair Value Measurements - Summary of Changes in Level 3 Fair Value Category (Detail) (USD $) In Thousands, unless otherwise specified	1 Months Ended	3 Months Ended	4 Months Ended	6 Months Ended	1 Months Ended	3 Months Ended	4 Months Ended	6 Months Ended	1 Months Ended	3 Months Ended	4 Months Ended	6 Months Ended	1 Months Ended	3 Months Ended	4 Months Ended	6 Months Ended	1 Months Ended	3 Months Ended	4 Months Ended	6 Months Ended	1 Months Ended	3 Months Ended	4 Months Ended	6 Months Ended	1 Months Ended	3 Months Ended	4 Months Ended	6 Months Ended	1 Months Ended	3 Months Ended	4 Months Ended	6 Months Ended	1 Months Ended	3 Months Ended	4 Months Ended	6 Months Ended	2 Months Ended									1 Months Ended	3 Months Ended	4 Months Ended	6 Months Ended	1 Months Ended	3 Months Ended	4 Months Ended	6 Months Ended	1 Months Ended	3 Months Ended	4 Months Ended	6 Months Ended	1 Months Ended	3 Months Ended	4 Months Ended	6 Months Ended	1 Months Ended	3 Months Ended	4 Months Ended	6 Months Ended	1 Months Ended	3 Months Ended	4 Months Ended	6 Months Ended	1 Months Ended	3 Months Ended	4 Months Ended	6 Months Ended	2 Months Ended
	Apr. 30, 2013 Predecessor Ambac [Member] Investment by class [Member]	Jun. 30, 2012 Predecessor Ambac [Member] Investment by class [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Investment by class [Member]	Jun. 30, 2012 Predecessor Ambac [Member] Investment by class [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Investment by class [Member] Other Asset Backed Securities [Member]	Jun. 30, 2012 Predecessor Ambac [Member] Investment by class [Member] Other Asset Backed Securities [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Investment by class [Member] Other Asset Backed Securities [Member]	Jun. 30, 2012 Predecessor Ambac [Member] Investment by class [Member] Other Asset Backed Securities [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Investment by class [Member] U.S. Agency Obligations [Member]	Jun. 30, 2012 Predecessor Ambac [Member] Investment by class [Member] U.S. Agency Obligations [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Investment by class [Member] U.S. Agency Obligations [Member]	Jun. 30, 2012 Predecessor Ambac [Member] Investment by class [Member] U.S. Agency Obligations [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Investment by class [Member] Collateralized Debt Obligations [Member]	Jun. 30, 2012 Predecessor Ambac [Member] Investment by class [Member] Collateralized Debt Obligations [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Investment by class [Member] Collateralized Debt Obligations [Member]	Jun. 30, 2012 Predecessor Ambac [Member] Investment by class [Member] Collateralized Debt Obligations [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Investment by class [Member] Corporate Obligations [Member]	Jun. 30, 2012 Predecessor Ambac [Member] Investment by class [Member] Corporate Obligations [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Investment by class [Member] Corporate Obligations [Member]	Jun. 30, 2012 Predecessor Ambac [Member] Investment by class [Member] Corporate Obligations [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Derivatives by class [Member]	Jun. 30, 2012 Predecessor Ambac [Member] Derivatives by class [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Derivatives by class [Member]	Jun. 30, 2012 Predecessor Ambac [Member] Derivatives by class [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Derivatives by class [Member] Interest Rate Swaps [Member]	Jun. 30, 2012 Predecessor Ambac [Member] Derivatives by class [Member] Interest Rate Swaps [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Derivatives by class [Member] Interest Rate Swaps [Member]	Jun. 30, 2012 Predecessor Ambac [Member] Derivatives by class [Member] Interest Rate Swaps [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Derivatives by class [Member] Credit Derivatives [Member]	Jun. 30, 2012 Predecessor Ambac [Member] Derivatives by class [Member] Credit Derivatives [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Derivatives by class [Member] Credit Derivatives [Member]	Jun. 30, 2012 Predecessor Ambac [Member] Derivatives by class [Member] Credit Derivatives [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Derivatives by class [Member] Call options on long-term debt [Member]	Jun. 30, 2012 Predecessor Ambac [Member] Derivatives by class [Member] Call options on long-term debt [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Derivatives by class [Member] Call options on long-term debt [Member]	Jun. 30, 2012 Predecessor Ambac [Member] Derivatives by class [Member] Call options on long-term debt [Member]	Jun. 30, 2013 Successor Ambac [Member] Investment by class [Member]	Jun. 30, 2013 Successor Ambac [Member] Investment by class [Member] Other Asset Backed Securities [Member]	Jun. 30, 2013 Successor Ambac [Member] Investment by class [Member] U.S. Agency Obligations [Member]	Jun. 30, 2013 Successor Ambac [Member] Investment by class [Member] Collateralized Debt Obligations [Member]	Jun. 30, 2013 Successor Ambac [Member] Investment by class [Member] Corporate Obligations [Member]	Jun. 30, 2013 Successor Ambac [Member] Derivatives by class [Member]	Jun. 30, 2013 Successor Ambac [Member] Derivatives by class [Member] Interest Rate Swaps [Member]	Jun. 30, 2013 Successor Ambac [Member] Derivatives by class [Member] Credit Derivatives [Member]	Jun. 30, 2013 Successor Ambac [Member] Derivatives by class [Member] Call options on long-term debt [Member]	Apr. 30, 2013 Level 3 [Member] Predecessor Ambac [Member]	Jun. 30, 2012 Level 3 [Member] Predecessor Ambac [Member]	Apr. 30, 2013 Level 3 [Member] Predecessor Ambac [Member]	Jun. 30, 2012 Level 3 [Member] Predecessor Ambac [Member]	Apr. 30, 2013 Level 3 [Member] Predecessor Ambac [Member] Investments contract [Member]	Jun. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] Investments contract [Member]	Apr. 30, 2013 Level 3 [Member] Predecessor Ambac [Member] Investments contract [Member]	Jun. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] Investments contract [Member]	Apr. 30, 2013 Level 3 [Member] Predecessor Ambac [Member] Loans [Member]	Jun. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] Loans [Member]	Apr. 30, 2013 Level 3 [Member] Predecessor Ambac [Member] Loans [Member]	Jun. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] Loans [Member]	Apr. 30, 2013 Level 3 [Member] Predecessor Ambac [Member] Long-term debt [Member]	Jun. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] Long-term debt [Member]	Apr. 30, 2013 Level 3 [Member] Predecessor Ambac [Member] Long-term debt [Member]	Jun. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] Long-term debt [Member]	Apr. 30, 2013 Level 3 [Member] Predecessor Ambac [Member] Investments [Member]	Jun. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] Investments [Member]	Apr. 30, 2013 Level 3 [Member] Predecessor Ambac [Member] Investments [Member]	Jun. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] Investments [Member]	Apr. 30, 2013 Level 3 [Member] Predecessor Ambac [Member] Other Assets [Member]	Jun. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] Other Assets [Member]	Apr. 30, 2013 Level 3 [Member] Predecessor Ambac [Member] Other Assets [Member]	Jun. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] Other Assets [Member]	Apr. 30, 2013 Level 3 [Member] Predecessor Ambac [Member] Derivatives assets [Member]	Jun. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] Derivatives assets [Member]	Apr. 30, 2013 Level 3 [Member] Predecessor Ambac [Member] Derivatives assets [Member]	Jun. 30, 2012 Level 3 [Member] Predecessor Ambac [Member] Derivatives assets [Member]	Jun. 30, 2013 Level 3 [Member] Successor Ambac [Member]	Jun. 30, 2013 Level 3 [Member] Successor Ambac [Member] Investments contract [Member]	Jun. 30, 2013 Level 3 [Member] Successor Ambac [Member] Loans [Member]	Jun. 30, 2013 Level 3 [Member] Successor Ambac [Member] Long-term debt [Member]	Jun. 30, 2013 Level 3 [Member] Successor Ambac [Member] Investments [Member]	Jun. 30, 2013 Level 3 [Member] Successor Ambac [Member] Other Assets [Member]	Jun. 30, 2013 Level 3 [Member] Successor Ambac [Member] Derivatives assets [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Balance, beginning of period	$ 57,791	$ 161,450	$ 60,402	$ 97,522	$ 50,234	$ 137,026	$ 50,264	$ 75,886		$ 1,218		$ 1,224	$ 3,905	$ 9,839	$ 6,482	$ 12,482	$ 3,652	$ 13,367	$ 3,656	$ 7,930	$ (328,689)	$ (355,616)	$ (322,337)	$ (486,775)	$ (125,382)	$ (222,222)	$ (108,752)	$ (302,177)	$ (203,307)	$ (201,129)	$ (213,585)	$ (190,653)		$ 67,735		$ 6,055	$ 69,412	$ 61,782		$ 3,949	$ 3,681	$ (415,360)	$ (137,947)	$ (277,413)		$ 14,364,161	$ 13,679,955	$ 14,416,488	$ 14,019,216	$ 2,414,607	$ 2,184,665	$ 2,261,294	$ 2,199,338	$ 14,116,811	$ 14,460,077	$ 15,359,073	$ 14,126,994	$ (1,910,589)	$ (2,786,644)	$ (2,956,501)	$ (1,934,642)	$ 57,791	$ 161,450	$ 60,402	$ 97,522	$ 14,230	$ 16,023	$ 14,557	$ 16,779	$ (328,689)	$ (355,616)	$ (322,337)	$ (486,775)	$ 15,170,359	$ 2,500,565	$ 14,752,053	$ (1,750,372)	$ 69,412	$ 14,061	$ (415,360)
Additions of VIEs consolidated																																														(409,300)		(409,300)										(409,300)		(409,300)
Included in earnings	(10)	88	(33)	40						(2)		(2)	(3)		(6)	(2)	(7)	90	(27)	44	(85,384)	(81,290)	(88,546)	41,037	(12,213)	(112,904)	(28,162)	(45,035)	(73,171)	(7,416)	(60,384)	(14,638)		39,030		100,710	395	392		18	(15)	70,127	18,907	51,220		490,336	91,712	1,057,181	160,521	31,474	20,103	328,768	(62,713)	555,946	131,807	956,402	298,472	(11,521)	21,235	(138,914)	(115,328)	(10)	88	(33)	40	(169)	(231)	(496)	(987)	(85,384)	(81,290)	(88,546)	41,037	(720,947)	(208,998)	(586,641)	4,347	395	(177)	70,127
Included in other comprehensive income	11,631	(1,588)	12,329	6,225	11,548	(1,696)	12,117	6,376		(3)		(9)	47	190	160	291	36	(79)	52	(433)																	63	319		(11)	(245)					128,819	(256,974)	(776,014)	171,634	54,484	(42,706)	(89,497)	25,437	105,622	(267,190)	(849,833)	148,667	(42,918)	54,510	150,987	(8,695)	11,631	(1,588)	12,329	6,225									(302,722)	(48,578)	(282,421)	28,214	63
Purchases
Issuances
Sales
Settlements		(89,879)	(3,286)	(92,621)		(84,584)	(599)	(84,584)		(749)		(749)		(471)	(2,687)	(3,213)		(4,075)		(4,075)	(1,287)	(70,698)	(4,477)	(61,866)	(352)	39,139	(1,033)	51,225	(935)	(3,072)	(3,444)	(6,326)		(106,765)		(106,765)	(1,298)	(757)		(541)		4,570	10,644	(6,074)		(27,613)	(227,135)	(716,488)	(457,454)					(26,326)	(86,421)	(713,589)	(335,860)		19,863	4,864	32,893		(89,879)	(3,286)	(92,621)					(1,287)	(70,698)	(4,477)	(61,866)	(58,841)		(62,113)		(1,298)		4,570
Transfers in Level 3				58,905				53,068												5,837																											(312,083)		(918,442)										(312,083)		(977,347)				58,905									(220,922)			(220,922)
Transfers out of Level 3		(6,301)		(6,301)						(464)		(464)						(5,837)		(5,837)																										623,956	650,758	1,598,492	650,758									623,956	657,059	1,598,492	657,059		(6,301)		(6,301)									365,046			365,046
Deconsolidation of VIEs
Balance, end of period	69,412	63,770	69,412	63,770	61,782	50,746	61,782	50,746					3,949	9,558	3,949	9,558	3,681	3,466	3,681	3,466	(415,360)	(507,604)	(415,360)	(507,604)	(137,947)	(295,987)	(137,947)	(295,987)	(277,413)	(211,617)	(277,413)	(211,617)					68,572	61,736		3,415	3,421	(340,663)	(108,396)	(232,267)		15,170,359	13,626,233	15,170,359	13,626,233	2,500,565	2,162,062	2,500,565	2,162,062	14,752,053	14,238,273	14,752,053	14,238,273	(1,750,372)	(2,346,060)	(1,750,372)	(2,346,060)	69,412	63,770	69,412	63,770	14,061	15,792	14,061	15,792	(415,360)	(507,604)	(415,360)	(507,604)	14,231,973	2,242,989	13,820,878	(1,573,687)	68,572	13,884	(340,663)
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date																					$ (86,320)	$ (118,474)	$ (92,002)	$ (72,910)	$ (12,213)	$ (107,861)	$ (28,162)	$ (46,263)	$ (74,107)	$ (10,613)	$ (63,840)	$ (26,647)										$ 32,525	$ 18,907	$ 13,618		$ 489,410	$ 54,440	$ 1,053,758	$ 47,015	$ 31,474	$ 20,103	$ 328,768	$ (62,713)	$ 555,946	$ 131,807	$ 956,402	$ 298,953	$ (11,521)	$ 21,235	$ (138,914)	$ (115,328)					$ (169)	$ (231)	$ (496)	$ (987)	$ (86,320)	$ (118,474)	$ (92,002)	$ (72,910)	$ (758,944)	$ (208,998)	$ (586,641)	$ 4,347		$ (177)	$ 32,525