Fresh Start Financial Statement Reporting - Fresh Start Reorganized Condensed Consolidated Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Apr. 30, 2013	Sep. 30, 2013 Successor Ambac [Member]	Jun. 30, 2013 Successor Ambac [Member]	Apr. 30, 2013 Successor Ambac [Member]	Sep. 30, 2013 Successor Ambac [Member] Variable Interest Entity [Member]	Apr. 30, 2013 Successor Ambac [Member] Variable Interest Entity [Member]	Apr. 30, 2013 Successor Ambac [Member] Common Stock-Predecessor Ambac [Member]	Apr. 30, 2013 Successor Ambac [Member] Common Stock Successor Ambac [Member]	Apr. 30, 2013 Successor Ambac [Member] Additional Paid-In Capital-Predecessor Ambac [Member]	Apr. 30, 2013 Successor Ambac [Member] Additional Paid-In Capital Successor [Member]	Apr. 30, 2013 Predecessor Ambac [Member]	Dec. 31, 2012 Predecessor Ambac [Member]	Sep. 30, 2012 Predecessor Ambac [Member]	Dec. 31, 2011 Predecessor Ambac [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Variable Interest Entity [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Variable Interest Entity [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Common Stock-Predecessor Ambac [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Common Stock Successor Ambac [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Additional Paid-In Capital-Predecessor Ambac [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Additional Paid-In Capital Successor [Member]	Apr. 30, 2013 Reorganization Items Adjustments [Member]	Apr. 30, 2013 Reorganization Items Adjustments [Member] Common Stock Successor Ambac [Member]	Apr. 30, 2013 Reorganization Items Adjustments [Member] Additional Paid-In Capital Successor [Member]	Apr. 30, 2013 Fresh Start Adjustments [Member]	Apr. 30, 2013 Fresh Start Adjustments [Member] Variable Interest Entity [Member]	Apr. 30, 2013 Fresh Start Adjustments [Member] Common Stock-Predecessor Ambac [Member]	Apr. 30, 2013 Fresh Start Adjustments [Member] Additional Paid-In Capital-Predecessor Ambac [Member]	Apr. 30, 2013 Fresh Start Adjustments [Member] Additional Paid-In Capital Successor [Member]
Assets:
Investments		$ 6,685,613		$ 6,457,264							$ 6,457,264	$ 6,329,932
Cash		28,551		152,951							254,851	43,837									(101,900)
Receivable for securities sold		19,339		682							682	761
Investment income due and accrued		34,115		37,961	4,154	4,851					37,961	39,742			4,851	4,101
Premium receivables		1,436,010		1,531,631							1,531,631	1,620,621
Reinsurance recoverable on paid and unpaid losses		137,400		151,311							151,311	159,086
Deferred ceded premium		152,076		166,212							166,212	177,893
Subrogation recoverable		484,605		533,673							533,673	497,346
Deferred acquisition costs											184,953	199,160												(184,953)
Loans		6,148		7,282							8,857	9,203												(1,575)
Derivative assets		82,962		121,643							121,643	126,106
Current taxes		3,589		4,410																	4,410
Insurance intangible asset		1,620,952		1,658,972																				1,658,972
Goodwill	514,511	514,511		514,511																				514,511
Other assets		41,349		54,821	13,354	13,972					54,821	39,715			13,972	5,467
Variable interest entity assets:
Fixed income securities, at fair value		5,758,665			2,423,914	2,500,565						5,402,395			2,500,565	2,261,294
Restricted cash					23,045	24,150									24,150	2,290
Investment income due and accrued		34,115		37,961	4,154	4,851					37,961	39,742			4,851	4,101
Loans					14,762,446	14,752,053									14,758,077	15,568,711									(6,024)
Intangible assets					159,505	164,520									164,520
Other assets		41,349		54,821	13,354	13,972					54,821	39,715			13,972	5,467
Total assets	28,853,435	28,633,638		28,853,435							28,853,435	27,085,265	26,947,892								(97,490)			1,980,931
Liabilities:
Liabilities subject to compromise											1,704,641	1,704,904									(1,704,641)
Unearned premiums		2,303,627		2,482,314							2,482,314	2,778,401
Losses and loss expense reserve		5,884,660		6,106,345							6,106,345	6,619,486
Ceded premiums payable		86,028		92,468							92,468	94,527
Obligations under investment agreements		359,484		358,878							357,373	356,091												1,505
Obligations under investment repurchase agreements		1,370		5,926							5,926	5,926
Deferred taxes		1,611		1,580							1,580	1,586
Current taxes											97,490	96,778									(97,490)
Long-term debt		954,721		940,313	15,266,893	15,023,038					155,271	150,170			15,041,624	15,436,008					(973)			786,015	(18,586)
Accrued interest payable		271,330		233,876	3,565	4,318					252,788	228,835			4,318	3,618					(821)			(18,091)
Derivative liabilities		400,382		621,645	2,047,204	2,425,517					621,645	531,315			2,425,517	2,221,781
Other liabilities		77,270		90,745	6,599	6,030					88,908	102,488			6,030	293								1,837
Payable for securities purchased		82,144		27							27	25
Variable interest entity liabilities:
Accrued interest payable		271,330		233,876	3,565	4,318					252,788	228,835			4,318	3,618					(821)			(18,091)
Long-term debt		954,721		940,313	15,266,893	15,023,038					155,271	150,170			15,041,624	15,436,008					(973)			786,015	(18,586)
Derivative liabilities		400,382		621,645	2,047,204	2,425,517					621,645	531,315			2,425,517	2,221,781
Other liabilities		77,270		90,745	6,599	6,030					88,908	102,488			6,030	293								1,837
Total liabilities	28,393,020	27,746,888		28,393,020							29,444,265	30,332,232									(1,803,925)			752,680
Stockholders' (deficit) equity:
Preferred stock
Common stock		450						450				3,080					3,080					450				(3,080)
Additional paid-in capital		184,566								184,550		2,172,027							2,172,027				184,550				(2,172,027)
Accumulated other comprehensive income		(10,246)	(103,507)								800,260	625,385												(800,260)
Accumulated deficit		436,656									(5,697,961)	(6,297,264)									1,521,435			4,176,526
Common stock held in treasury at cost											(410,695)	(410,755)												410,695
Total Ambac Financial Group, Inc. stockholders' (deficit) equity		611,426		185,000							(3,133,289)	(3,907,527)									1,706,435			1,611,854
Noncontrolling interest	275,415	275,324		275,415							659,018	660,560												(383,603)
Total stockholders' (deficit) equity		886,750		460,415							275,415	(3,246,967)	(3,487,513)	(3,149,533)							1,706,435			1,228,251
Total liabilities and stockholders' (deficit) equity		$ 28,633,638		$ 28,853,435							$ 26,969,994	$ 27,085,265									$ (97,490)			$ 1,980,931