Schedule II - Condensed Financial Information of Registrant (Parent Company Only) - Condensed Statement of Total Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	2 Months Ended	3 Months Ended						8 Months Ended	12 Months Ended	1 Months Ended	3 Months Ended	4 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2013	Dec. 31, 2014	Apr. 30, 2013	Mar. 31, 2013	Apr. 30, 2013	Dec. 31, 2012
Successor [Member]
Revenues:
Other income								$ 4,364	$ 12,498
Net realized investment gains								(4,467)	(58,777)
Total revenues before expenses and reorganization items								581,048	328,670
Expenses:
Reorganization items	424	0	2	186	23	65	4	493	211
Income (loss) before income taxes and equity in undistributed net loss of subsidiaries	205,795	459,594	84,773	(210,412)	159,298	74,920	231,601	512,316	493,253
Income tax expense (benefit)								(7,514)	(9,557)
Net income (loss)								504,802	483,696
Other comprehensive income, after tax:
Net income (loss)								504,802	483,696
Unrealized gains (losses) on securities, net of deferred income taxes of $0								(41,910)	252,603
Gains (losses) on foreign currency translation, net of deferred income taxes of $0								43,165	(43,599)
Total other comprehensive income								12,102	208,188
Total comprehensive income (loss) attributable to Ambac Financial Group, Inc.								516,880	692,693
Predecessor [Member]
Revenues:
Other income												8,363	106,098
Net realized investment gains												(53,305)	(72,101)
Total revenues before expenses and reorganization items												640,388	685,689
Expenses:
Reorganization items										(2,747,239)	2,059	(2,745,180)	7,215
Income (loss) before income taxes and equity in undistributed net loss of subsidiaries										3,065,124	282,909	3,348,033	(256,505)
Income tax expense (benefit)												(755)	(2,849)
Net income (loss)												3,347,278	(259,354)
Other comprehensive income, after tax:
Net income (loss)												3,347,278	(259,354)
Unrealized gains (losses) on securities, net of deferred income taxes of $0												175,347	166,252
Gains (losses) on foreign currency translation, net of deferred income taxes of $0												(428)	(540)
Total other comprehensive income												175,104	161,920
Total comprehensive income (loss) attributable to Ambac Financial Group, Inc.												3,523,924	(94,552)
Ambac Financial Group, Inc Parent Company Only [Member] | Successor [Member]
Revenues:
Interest income								22,227	25,147
Other income								197,122	0
Net realized investment gains								(2)	46
Total revenues before expenses and reorganization items								219,351	25,101
Expenses:
Operating expenses								3,018	6,458
Total expenses								3,018	6,458
Income (loss) before income taxes, reorganization costs and equity in undistributed net loss of subsidiaries								216,333	18,643
Reorganization items								493	211
Income (loss) before income taxes and equity in undistributed net loss of subsidiaries								215,840	18,432
Income tax expense (benefit)								0	(221)
Income (loss) before equity in undistributed net income (loss) of subsidiaries								215,840	18,211
Equity in undistributed net income (loss) of subsidiaries								289,379	465,860
Net income (loss)								505,219	484,071
Other comprehensive income, after tax:
Net income (loss)								505,219	484,071
Unrealized gains (losses) on securities, net of deferred income taxes of $0								(41,910)	252,603
Gains (losses) on foreign currency translation, net of deferred income taxes of $0								42,724	(43,165)
Changes to postretirement benefit, net of tax								10,847	(816)
Total other comprehensive income								11,661	208,622
Total comprehensive income (loss) attributable to Ambac Financial Group, Inc.								516,880	692,693
Ambac Financial Group, Inc Parent Company Only [Member] | Predecessor [Member]
Revenues:
Interest income												39	221
Other income												0	758
Net realized investment gains												0	0
Total revenues before expenses and reorganization items												39	979
Expenses:
Operating expenses												539	(151)
Total expenses												539	(151)
Income (loss) before income taxes, reorganization costs and equity in undistributed net loss of subsidiaries												(500)	1,130
Reorganization items												(2,745,180)	7,215
Income (loss) before income taxes and equity in undistributed net loss of subsidiaries												2,744,680	(6,085)
Income tax expense (benefit)												703	136
Income (loss) before equity in undistributed net income (loss) of subsidiaries												2,745,383	(5,949)
Equity in undistributed net income (loss) of subsidiaries												603,666	(250,729)
Net income (loss)												3,349,049	(256,678)
Other comprehensive income, after tax:
Net income (loss)												3,349,049	(256,678)
Unrealized gains (losses) on securities, net of deferred income taxes of $0												175,347	166,252
Gains (losses) on foreign currency translation, net of deferred income taxes of $0												(657)	(334)
Changes to postretirement benefit, net of tax												185	(3,792)
Total other comprehensive income												174,875	162,126
Total comprehensive income (loss) attributable to Ambac Financial Group, Inc.												$ 3,523,924	$ (94,552)