Schedule II - Condensed Financial Information of Registrant (Parent Company Only) - Condensed Statement of Total Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Income Statements, Captions [Line Items]
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, Tax											$ 0
Revenues:
Other income	$ 1,000	$ 141,000	$ (9,000)	$ 1,000	$ 2,000	$ 1,000	$ 2,000	$ (1,000)	$ 134,000	$ 5,000	0
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net	0	0	0	0	(2,000)	0	(1,000)	0	0	(3,000)	(20,000)
Net realized investment gains	(9,000)	(18,000)	(36,000)	(17,000)	(29,000)	(30,000)	(47,000)	(5,000)	(81,000)	(112,000)	(5,000)
Total revenues									496,000	511,000	622,000
Expenses:
Pre-tax income (loss)	(111,000)	69,000	(100,000)	(41,000)	(22,000)	(20,000)	6,000	308,000	(183,000)	273,000	(284,000)
Income tax expense (benefit)									(32,000)	(5,000)	(44,000)
Net income (loss) attributable to common stockholders	(110,000)	66,000	(128,000)	(43,000)	(20,000)	(104,000)	4,000	306,000	(216,000)	186,000	(329,000)
Other comprehensive income, after tax:
Net income (loss)	$ (110,000)	$ 66,000	$ (128,000)	$ (43,000)	$ (20,000)	$ (22,000)	$ 4,000	$ 306,000	(216,000)	267,000	(329,000)
Unrealized gains (losses) on securities, net of deferred income taxes of $0									65,000	55,000	(82,000)
Gains (losses) on foreign currency translation, net of income tax provision (benefit) of $0, $0 and $0									26,000	(48,000)	74,000
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, after Tax									0	1,000	0
Total other comprehensive income (loss), net of income tax									91,000	6,000	(7,000)
Total comprehensive income (loss) attributable to common stockholders									(125,000)	192,000	(335,000)
Ambac Financial Group, Inc Parent Company Only [Member]
Condensed Income Statements, Captions [Line Items]
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax									(8,000)	2,000	0
Other Comprehensive Income (Loss), Foreign Currency Translation Gain (Loss) Arising During Period, Tax									0	0	0
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, Tax									0	0
Revenues:
Investment income									19,000	28,000	24,000
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net									(2,000)	(1,000)	(1,000)
Net realized investment gains									(1,000)	1,000	7,000
Total revenues									18,000	26,000	17,000
Expenses:
Operating Expenses									16,000	8,000	4,000
Total expenses									16,000	8,000	4,000
Pre-tax income (loss)									2,000	17,000	13,000
Income tax expense (benefit)									5,000	11,000	29,000
Income before equity in undistributed net income (loss) of subsidiaries									(7,000)	(28,000)	(43,000)
Equity in undistributed net income (loss) of subsidiaries									(223,000)	157,000	(371,000)
Net income (loss) attributable to common stockholders									(216,000)	186,000	(329,000)
Other comprehensive income, after tax:
Net income (loss)									(216,000)	186,000	(329,000)
Unrealized gains (losses) on securities, net of deferred income taxes of $0									65,000	55,000	(82,000)
Gains (losses) on foreign currency translation, net of income tax provision (benefit) of $0, $0 and $0									26,000	(48,000)	74,000
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, after Tax									0	1,000	0
Changes to postretirement benefit, net of tax									(1,000)	(2,000)	1,000
Total other comprehensive income (loss), net of income tax									91,000	6,000	(7,000)
Total comprehensive income (loss) attributable to common stockholders									(125,000)	192,000	(335,000)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Plan Amendments, Tax Effect									$ 0	$ 0	$ 0