Fresh Start Financial Statement Reporting - Fresh Start Reorganized Condensed Consolidated Balance Sheet (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	May. 01, 2013	Apr. 30, 2013	Dec. 31, 2012
Assets:
Goodwill					$ 514,511
Variable interest entity assets:
Total assets					28,853,435
Liabilities:
Liabilities subject to compromise					1,704,641
Variable interest entity liabilities:
Total liabilities					28,393,020
Stockholders' (deficit) equity:
Common stock					450
Additional paid-in capital					184,550
Noncontrolling interest					$ 275,415
Common stock, shares issued					45,000,000
Predecessor [Member]
Fresh-Start Adjustment [Line Items]
Assets unadjusted					$ 26,969,994
Assets:
Investments					6,457,264
Cash					254,851
Receivable for securities sold					682
Investment income due and accrued					37,961
Premium receivables					1,531,631
Reinsurance recoverable on paid and unpaid losses					151,311
Deferred ceded premium					166,212
Subrogation recoverable					533,673
Deferred acquisition costs					184,953
Loans					8,857
Derivative assets					121,643
Current taxes					0
Insurance intangible asset					0
Goodwill					0
Other assets					54,821
Variable interest entity assets:
Investment income due and accrued					37,961
Loans					8,857
Other assets					54,821
Total assets					28,853,435
Liabilities:
Liabilities subject to compromise					1,704,641
Unearned premiums					2,482,314
Loss and loss expense reserves					6,106,345
Ceded premiums payable					92,468
Obligations under investment agreements					357,373
Obligations under investment repurchase agreements					5,926
Deferred taxes					1,580
Current taxes					97,490
Long-term debt					155,271
Accrued interest payable					252,788
Derivative liabilities					621,645
Other liabilities					88,908
Payable for securities purchased					27
Variable interest entity liabilities:
Accrued interest payable					252,788
Long-term debt					155,271
Derivative liabilities					621,645
Other liabilities					88,908
Total liabilities					29,444,265
Stockholders' (deficit) equity:
Preferred stock					0
Accumulated other comprehensive income					800,260
Retained earnings					(5,697,961)
Common stock held in treasury at cost					410,695
ambc_stockholdersequityattributabletoparentunadjusted					(3,133,289)
ambc_stockholdersequityattributabletononcontrollinginterestunadjusted					659,018
ambc_stockholdersequityincludingportionattributabletononcontrollinginterestUnadjusted					(2,474,271)
Total stockholders' (deficit) equity					275,415	$ (3,246,967)
Lliabilities and stockholders equity unadjusted					26,969,994
Predecessor [Member] | Variable Interest Entity [Member]
Assets:
Investment income due and accrued					4,851
Loans					14,758,077
Other assets					13,972
Variable interest entity assets:
Fixed income securities, at fair value					2,500,565
Restricted cash					24,150
Investment income due and accrued					4,851
Loans					14,758,077
Intangible assets					164,520
Other assets					13,972
Liabilities:
Long-term debt					15,041,624
Accrued interest payable					4,318
Derivative liabilities					2,425,517
Other liabilities					6,030
Variable interest entity liabilities:
Accrued interest payable					4,318
Long-term debt					15,041,624
Derivative liabilities					2,425,517
Other liabilities					6,030
Predecessor [Member] | Common Stock-Predecessor Ambac [Member]
Stockholders' (deficit) equity:
Common stock					3,080
Predecessor [Member] | Common Stock Successor Ambac [Member]
Stockholders' (deficit) equity:
Common stock					0
Predecessor [Member] | Additional Paid-In Capital-Predecessor Ambac [Member]
Stockholders' (deficit) equity:
Additional paid-in capital					2,172,027
Predecessor [Member] | Additional Paid-In Capital Successor [Member]
Stockholders' (deficit) equity:
Additional paid-in capital					0
Successor [Member]
Assets:
Investments	$ 5,644,720	$ 5,507,034			6,457,264
Cash					152,951
Receivable for securities sold	44,030	23,660			682
Investment income due and accrued	25,264	25,015			37,961
Premium receivables	831,575	1,000,607			1,531,631
Reinsurance recoverable on paid and unpaid losses	43,999	99,838			151,311
Deferred ceded premium	96,758	123,276			166,212
Subrogation recoverable	1,229,293	953,274			533,673
Deferred acquisition costs					0
Loans	5,206	5,714			7,282
Derivative assets	84,995	109,017			121,643
Current taxes					4,410
Insurance intangible asset	1,212,112	1,410,920			1,658,972
Goodwill	0	514,511	$ 514,511		514,511
Other assets	185,877	186,985			54,821
Variable interest entity assets:
Fixed income securities, at fair value	5,043,776	4,725,686
Investment income due and accrued	25,264	25,015			37,961
Loans	5,206	5,714			7,282
Other assets	185,877	186,985			54,821
Total assets	23,728,070	25,159,864	27,092,477		28,853,435
Liabilities:
Liabilities subject to compromise					0
Unearned premiums	1,280,282	1,673,785			2,482,314
Loss and loss expense reserves	4,088,106	4,752,007			6,106,345
Ceded premiums payable	53,494	60,436			92,468
Obligations under investment agreements	100,358	160,079			358,878
Obligations under investment repurchase agreements					5,926
Deferred taxes					1,580
Current taxes	5,835	5,701			0
Long-term debt	1,124,950	971,116			940,313
Accrued interest payable	355,536	304,139			233,876
Derivative liabilities	353,358	406,944			621,645
Other liabilities	61,134	63,396			90,745
Payable for securities purchased	84,690	762			27
Variable interest entity liabilities:
Accrued interest payable	355,536	304,139			233,876
Long-term debt	1,124,950	971,116			940,313
Derivative liabilities	353,358	406,944			621,645
Other liabilities	61,134	63,396			90,745
Total liabilities	21,769,724	23,486,129			28,393,020
Stockholders' (deficit) equity:
Preferred stock	0	0			0
Common stock	450	450			450
Additional paid-in capital	190,813	189,138
Accumulated other comprehensive income	15,215	220,283	11,661		0
Retained earnings	1,478,439	989,290			0
Common stock held in treasury at cost	(118)	(56)			0
Total Ambac Financial Group, Inc. stockholders’ equity	1,684,799	1,399,105			185,000
Noncontrolling interest	273,547	274,630			275,415
Total stockholders' (deficit) equity	1,958,346	1,673,735	$ 978,422	$ 460,415	460,415
Total liabilities and stockholders’ equity	$ 23,728,070	$ 25,159,864			28,853,435
Common stock, shares issued	45,044,222	45,005,932		45,000,000
Successor [Member] | Variable Interest Entity [Member]
Assets:
Investment income due and accrued	$ 1,213	$ 1,284			4,851
Loans					14,752,053
Other assets	2,582	2,891			13,972
Variable interest entity assets:
Fixed income securities, at fair value	2,588,556	2,743,050			2,500,565
Restricted cash	5,822	7,708			24,150
Investment income due and accrued	1,213	1,284			4,851
Loans					14,752,053
Intangible assets					164,520
Other assets	2,582	2,891			13,972
Liabilities:
Long-term debt	12,327,960	12,882,076			15,023,038
Accrued interest payable	3,230	3,268			4,318
Derivative liabilities	1,928,403	2,200,163			2,425,517
Other liabilities	183	178			6,030
Variable interest entity liabilities:
Accrued interest payable	3,230	3,268			4,318
Long-term debt	12,327,960	12,882,076			15,023,038
Derivative liabilities	1,928,403	2,200,163			2,425,517
Other liabilities	$ 183	$ 178			6,030
Successor [Member] | Common Stock-Predecessor Ambac [Member]
Stockholders' (deficit) equity:
Common stock					0
Successor [Member] | Common Stock Successor Ambac [Member]
Stockholders' (deficit) equity:
Common stock					450
Successor [Member] | Additional Paid-In Capital-Predecessor Ambac [Member]
Stockholders' (deficit) equity:
Additional paid-in capital					0
Successor [Member] | Additional Paid-In Capital Successor [Member]
Stockholders' (deficit) equity:
Additional paid-in capital					184,550
Reorganization Item Adjustments [Member]
Assets:
Cash					(101,900)
Current taxes					4,410
Variable interest entity assets:
Total assets					(97,490)
Liabilities:
Liabilities subject to compromise					(1,704,641)
Current taxes					(97,490)
Long-term debt					(973)
Accrued interest payable					(821)
Variable interest entity liabilities:
Accrued interest payable					(821)
Long-term debt					(973)
Total liabilities					(1,803,925)
Stockholders' (deficit) equity:
Preferred stock					0
Retained earnings					1,521,435
Total Ambac Financial Group, Inc. stockholders’ equity					1,706,435
Total stockholders' (deficit) equity					1,706,435
Total liabilities and stockholders’ equity					(97,490)
Reorganization Item Adjustments [Member] | Common Stock Successor Ambac [Member]
Stockholders' (deficit) equity:
Common stock					450
Reorganization Item Adjustments [Member] | Additional Paid-In Capital Successor [Member]
Stockholders' (deficit) equity:
Additional paid-in capital					184,550
Fresh Start Adjustments [Member]
Assets:
Deferred acquisition costs					(184,953)
Loans					(1,575)
Insurance intangible asset					1,658,972
Goodwill					514,511
Variable interest entity assets:
Loans					(1,575)
Total assets					1,980,931
Liabilities:
Obligations under investment agreements					1,505
Long-term debt					786,015
Accrued interest payable					(18,091)
Other liabilities					1,837
Variable interest entity liabilities:
Accrued interest payable					(18,091)
Long-term debt					786,015
Other liabilities					1,837
Total liabilities					752,680
Stockholders' (deficit) equity:
Preferred stock					0
Accumulated other comprehensive income					(800,260)
Retained earnings					4,176,526
Common stock held in treasury at cost					(410,695)
Total Ambac Financial Group, Inc. stockholders’ equity					1,611,854
Noncontrolling interest					(383,603)
Total stockholders' (deficit) equity					1,228,251
Total liabilities and stockholders’ equity					1,980,931
Fresh Start Adjustments [Member] | Variable Interest Entity [Member]
Assets:
Loans					(6,024)
Variable interest entity assets:
Loans					(6,024)
Liabilities:
Long-term debt					(18,586)
Variable interest entity liabilities:
Long-term debt					(18,586)
Fresh Start Adjustments [Member] | Common Stock-Predecessor Ambac [Member]
Stockholders' (deficit) equity:
Common stock					(3,080)
Fresh Start Adjustments [Member] | Additional Paid-In Capital-Predecessor Ambac [Member]
Stockholders' (deficit) equity:
Additional paid-in capital					$ (2,172,027)
Fresh Start Adjustments [Member] | Additional Paid-In Capital Successor [Member]
Stockholders' (deficit) equity:
Additional paid-in capital