Schedule II - Condensed Financial Information of Registrant (Parent Company Only) - Condensed Statement of Total Comprehensive Income (Detail) - Successor [Member] - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Other income (expense)									$ (706)	$ 17,445	$ 7,150
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net	$ 956	$ 13,510	$ 1,763	$ 3,942	$ 2,191	$ 2,853	$ 7,441	$ 9,334	(20,171)	(21,819)	(25,659)
Net realized investment gains	68	(6,150)	(4,180)	4,896	(11,536)	(11,749)	(14,897)	(1,102)	(5,366)	(39,284)	(53,476)
Total revenues									621,246	506,149	644,658
Expenses:
Pre-tax income (loss)	$ (6,917)	$ (185,466)	$ 13,992	$ (105,860)	$ (86,059)	$ 116,720	$ 61,511	$ 12,854	(284,251)	105,026	510,058
Income tax expense (benefit)									(44,464)	(30,709)	(17,364)
Net income (loss)									(328,715)	74,317	492,694
Other comprehensive income, after tax:
Net income (loss)									(328,715)	74,317	492,694
Unrealized gains (losses) on securities, net of deferred income taxes of $0									(81,520)	67,900	(159,730)
Gains (losses) on foreign currency translation, net of deferred income taxes of $0									73,586	(122,128)	(45,025)
Total other comprehensive income (loss), net of tax									(6,661)	(54,205)	(205,442)
Total comprehensive income (loss) attributable to Ambac Financial Group, Inc.									(335,376)	20,638	288,335
Ambac Financial Group, Inc Parent Company Only [Member]
Revenues:
Investment income									24,411	13,493	9,826
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net									(550)	(289)	(155)
Net realized investment gains									6,575	7	27
Total revenues									17,286	13,197	9,644
Expenses:
Operating Expenses									3,913	11,486	8,922
Total expenses									3,913	11,486	8,922
Pre-tax income (loss)									13,373	1,711	722
Income tax expense (benefit)									29,398	28,739	70,811
Income before equity in undistributed net income of subsidiaries									(42,771)	(30,450)	(71,533)
Equity in undistributed net income (loss) of subsidiaries									(371,486)	44,393	421,870
Net income (loss)									(328,715)	74,843	493,403
Other comprehensive income, after tax:
Net income (loss)									(328,715)	74,843	493,403
Unrealized gains (losses) on securities, net of deferred income taxes of $0									(81,520)	67,900	(159,730)
Gains (losses) on foreign currency translation, net of deferred income taxes of $0									73,586	(122,128)	(44,651)
Changes to postretirement benefit, net of tax									1,273	23	(687)
Total other comprehensive income (loss), net of tax									(6,661)	(54,205)	(205,068)
Total comprehensive income (loss) attributable to Ambac Financial Group, Inc.									$ (335,376)	$ 20,638	$ 288,335