Fresh Start Financial Statement Reporting - Fresh Start Reorganized Condensed Consolidated Balance Sheet (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Apr. 30, 2013	Dec. 31, 2012	Dec. 31, 2011	May 01, 2013
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	45,005,932	45,003,461	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	0	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	27,085,265
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	625,385
Accumulated 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)	(3,149,533)
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 (amortized cost of $4,514,878 in 2014 and $5,927,254 in 2013)			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	5,507,034	6,523,727	6,457,264
Cash			152,951
Receivable for securities sold	23,660	14,450	682
Investment income due and accrued	25,015	37,663	37,961
Premium receivables	1,000,607	1,453,021	1,531,631
Reinsurance recoverable on paid and unpaid losses	99,838	121,249	151,311
Deferred ceded premium	123,276	145,529	166,212
Subrogation recoverable	953,274	498,478	533,673
Deferred acquisition costs			0
Loans	5,714	6,179	7,282
Derivative assets	109,017	77,711	121,643
Current taxes			4,410
Insurance intangible asset	1,410,920	1,597,965	1,658,972
Goodwill	514,511	514,511	514,511			0
Other assets	186,985	35,927	54,821
Variable interest entity assets:
Fixed income securities, at fair value (amortized cost of $4,514,878 in 2014 and $5,927,254 in 2013)	4,725,686	5,885,316
Investment income due and accrued	25,015	37,663	37,961
Loans	5,714	6,179	7,282
Other assets	186,985	35,927	54,821
Total assets	25,159,864	27,092,477	28,853,435
Liabilities:
Liabilities subject to compromise			0
Unearned premiums	1,673,785	2,255,680	2,482,314
Loss and loss expense reserves	4,752,007	5,968,712	6,106,345
Ceded premiums payable	60,436	70,962	92,468
Obligations under investment agreements	160,079	359,070	358,878
Obligations under investment repurchase agreements			5,926
Deferred taxes	2,079	2,199	1,580
Current taxes	5,701	738	0
Long-term debt	971,116	963,178	940,313
Accrued interest payable	304,139	294,817	233,876
Derivative liabilities	406,944	253,898	621,645
Other liabilities	63,396	67,377	90,745
Payable for securities purchased	762	4,654	27
Variable interest entity liabilities:
Accrued interest payable	304,139	294,817	233,876
Long-term debt	971,116	963,178	940,313
Derivative liabilities	406,944	253,898	621,645
Other liabilities	63,396	67,377	90,745
Total liabilities	23,486,129	26,114,055	28,393,020
Stockholders' (deficit) equity:
Preferred stock	0	0	0
Common stock	450	450	450
Additional paid-in capital	189,138	185,672
Accumulated other comprehensive income	220,283	11,661	0			0
Accumulated earnings	989,290	505,219	0
Common stock held in treasury at cost	(56)	(19)	0
Total Ambac Financial Group, Inc. stockholders’ equity	1,399,105	702,983	185,000
Noncontrolling interest	274,630	275,439	275,415
Total stockholders' (deficit) equity	1,673,735	978,422	460,415			460,415
Total liabilities and stockholders’ equity	25,159,864	27,092,477	28,853,435
Common stock, shares issued			45,000,000
Successor [Member] | Variable Interest Entity [Member]
Assets:
Investment income due and accrued			4,851
Loans			14,752,053
Other assets			13,972
Variable interest entity assets:
Fixed income securities, at fair value (amortized cost of $4,514,878 in 2014 and $5,927,254 in 2013)			2,500,565
Restricted cash			24,150
Investment income due and accrued			4,851
Loans			14,752,053
Intangible assets			164,520
Other assets			13,972
Liabilities:
Long-term debt			15,023,038
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			15,023,038
Derivative liabilities			2,425,517
Other liabilities			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
Accumulated 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)
Accumulated 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