Schedule II - Condensed Financial Information of Registrant (Parent Company Only) - Condensed Statement of Total Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues:
Other income	$ 3,000	$ 134,000	$ 5,000
Net realized investment gains	(22,000)	(81,000)	(108,000)
Total revenues	156,000	496,000	511,000
Expenses:
Pre-tax income (loss)	(440,000)	(183,000)	273,000
Income tax expense (benefit)	3,000	(32,000)	(5,000)
Net income (loss) attributable to common stockholders	(437,000)	(216,000)	186,000
Other comprehensive income, after tax:
Net income (loss)	(437,000)	(216,000)	267,000
Unrealized gains (losses) on securities, net of deferred income taxes of $0	15,000	65,000	55,000
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax	23,000	26,000	(48,000)
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, after Tax	1,000	0	1,000
Total other comprehensive income (loss), net of income tax	37,000	91,000	6,000
Total comprehensive income (loss) attributable to common stockholders	(400,000)	(125,000)	192,000
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	1,000	(8,000)	2,000
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	0
Revenues:
Investment income	13,000	(1,000)	(2,000)
Net realized investment gains	1,000	2,000	2,000
Total revenues	12,000	18,000	26,000
Expenses:
Operating Expenses	19,000	16,000	8,000
Pre-tax income (loss)	(7,000)	2,000	17,000
Income tax expense (benefit)	0	5,000	11,000
Income before equity in undistributed net income (loss) of subsidiaries	7,000	(7,000)	(28,000)
Equity in undistributed net income (loss) of subsidiaries	(430,000)	(223,000)	157,000
Net income (loss) attributable to common stockholders	(437,000)	(216,000)	186,000
Other comprehensive income, after tax:
Net income (loss)	(437,000)	(216,000)	186,000
Unrealized gains (losses) on securities, net of deferred income taxes of $0	15,000	65,000	55,000
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax	23,000	26,000	(48,000)
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, after Tax	1,000	0	1,000
Changes to postretirement benefit, net of tax	(3,000)	(1,000)	(2,000)
Total other comprehensive income (loss), net of income tax	37,000	91,000	6,000
Total comprehensive income (loss) attributable to common stockholders	(400,000)	(125,000)	192,000
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Plan Amendments, Tax Effect	$ 0	$ 0	$ 0