Schedule II - Condensed Financial Information of Registrant (Parent Company Only) - Condensed Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	May 01, 2013	Apr. 30, 2013	Dec. 31, 2012
Assets:
Junior surplus note of Ambac Assurance Segregated Account			$ 350,000
Other investments	336,013	240,969
Total assets				28,853,435
Liabilities:
Total liabilities				28,393,020
Stockholders' equity (deficit):
Common stock				450
Additional paid-in capital				184,550
Successor [Member]
Assets:
Available-for-sale Securities, Debt Securities	4,725,686	5,885,316
Other investments	357,016
Total investments	5,507,034	6,523,727		6,457,264
Cash				152,951
Investment income due and accrued	25,015	37,663		37,961
Other assets	186,985	35,927		54,821
Total assets	25,159,864	27,092,477		28,853,435
Liabilities:
Current taxes	5,701	738		0
Total liabilities	23,486,129	26,114,055		28,393,020
Stockholders' equity (deficit):
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
Retained earnings (accumulated deficit)	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
Total liabilities and stockholders’ equity	25,159,864	27,092,477		28,853,435
Predecessor [Member]
Assets:
Total investments				6,457,264
Cash				254,851
Investment income due and accrued				37,961
Other assets				54,821
Total assets				28,853,435	27,085,265
Liabilities:
Current taxes				97,490
Total liabilities				29,444,265
Stockholders' equity (deficit):
Preferred stock				0
Accumulated other comprehensive income				800,260	625,385
Retained earnings (accumulated deficit)				(5,697,961)
Common stock held in treasury at cost				410,695
Ambac Financial Group, Inc Parent Company Only [Member] | Successor [Member]
Assets:
Available-for-sale Securities, Debt Securities	234,310	28,145
Junior surplus note of Ambac Assurance Segregated Account	0	177,194
Short-term investments, at cost (approximates fair value)	22,936	8,319
Other investments	21,003	0
Total investments	278,249	213,658
Cash	141	43
Investment in subsidiaries	1,116,044	473,215
Investment income due and accrued	54	11,959
Other assets	5,770	4,936
Total assets	1,400,258	703,811
Liabilities:
Current taxes	221	0
Accounts payable and other liabilities	932	828
Total liabilities	1,153	828
Stockholders' equity (deficit):
Preferred stock	0	0
Common stock	450	450
Additional paid-in capital	189,138	185,672
Accumulated other comprehensive income	220,283	11,661
Retained earnings (accumulated deficit)	989,290	505,219
Common stock held in treasury at cost	(56)	(19)
Total Ambac Financial Group, Inc. stockholders’ equity	1,399,105	702,983
Total liabilities and stockholders’ equity	$ 1,400,258	$ 703,811