Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended								6 Months Ended	12 Months Ended			66 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2017
Proceeds From Issuance of Tier II Notes										$ 0	$ 0	$ 240,000
Cash flows from operating activities:
Net income (loss) attributable to common stockholders	$ (14,000)	$ (108,000)	$ (35,000)	$ (280,000)	$ (110,000)	$ 66,000	$ (128,000)	$ (43,000)		(437,000)	(216,000)	186,000
AMPS Exchange Loss Based on Carry Value										0	0	82,000
Net income (loss)										(437,000)	(216,000)	267,000
Adjustments to reconcile net income to net cash used in operating activities:
Depreciation and amortization										1,000	0	1,000
Amortization of bond premium and discount										(15,000)	(63,000)	(137,000)
Share-based compensation										11,000	12,000	12,000
Deferred income taxes										(9,000)	1,000	7,000
Current income taxes										17,000	35,000	(35,000)
Unearned premiums, net										(48,000)	(132,000)	(163,000)
Losses and loss expenses, net										76,000	(364,000)	(1,633,000)
Ceded premiums payable										(3,000)	(4,000)	(5,000)
Premium receivables										44,000	77,000	91,000
Accrued interest payable										93,000	87,000	9,000
Amortization of insurance intangible assets	16,000	14,000	14,000	13,000	15,000	17,000	226,000	36,000		57,000	295,000	107,000
Net realized investment gains	(2,000)	(2,000)	(10,000)	(8,000)	(9,000)	(18,000)	(36,000)	(17,000)		(22,000)	(81,000)	(108,000)
Net realized gains (losses) on extinguishment of debt										0	0	(3,000)
Other, net										59,000	79,000	68,000
Net cash used in operating activities										(175,000)	(311,000)	(1,543,000)
Cash flows from investing activities:
Proceeds from sales of bonds										1,109,000	1,212,000	1,248,000
Proceeds from matured bonds										137,000	379,000	432,000
Purchases of bonds										(975,000)	(959,000)	(528,000)
Proceeds from sales of other invested assets										374,000	81,000	159,000
Purchases of other invested assets										(475,000)	(137,000)	(140,000)
Change in short-term investments										158,000	(218,000)	127,000
Change in cash collateral receivable										0	100,000	(58,000)
Proceeds From Financial Guaranty Variable Interest Entity Assets										178,000	543,000	349,000
Payments to Acquire Businesses, Net of Cash Acquired										(74,000)
Other, net										1,000	(2,000)	0
Net cash provided by investing activities										432,000	1,000,000	1,588,000
Cash flows from financing activities:
Proceeds from Sale of Senior Surplus Notes										0	0	24,000
Paydown of Ambac Note										(121,000)	(178,000)	(214,000)
Paydown Of Variable Interest Entity Secured Borrowing										0	0	74,000
Payments for extinguishment of surplus notes										0	0	(191,000)
Payments of Debt Issuance Costs										0	0	(9,000)
AMPS Proceeds										0	19,000	0
AMPS Exchange Cash Paid									$ (11,000)	0	0	(11,000)
Payment, Tax Withholding, Share-based Payment Arrangement										(3,000)	(3,000)	(1,000)
Payments for Repurchase of Warrants													$ (8,000)
Paydown of Financial Guaranty Variable Interest Entity Liabilities										(178,000)	(542,000)	(349,000)
Net Cash Provided by (Used in) Financing Activities										(303,000)	(691,000)	(585,000)
Effect of Exchange Rate on Cash and Cash Equivalents										0	0	0
Cash and Cash Equivalents, Period Increase (Decrease)										(46,000)	(2,000)	(541,000)
Cash and cash equivalents end of period	20,000				24,000					20,000	24,000	63,000	20,000
Supplemental disclosure of cash flow information
Income Taxes Paid										11,000	21,000	35,000
Cash payments related to reorganization items:
Variable interest entity activities	(3,000)	$ 0	$ 0	$ (3,000)	(7,000)	$ (11,000)	$ (3,000)	$ (16,000)		(5,000)	(38,000)	(3,000)
Increase (Decrease) in Derivative Assets and Liabilities										6,000	(1,000)	(17,000)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	35,000				81,000					35,000	81,000	83,000	35,000	$ 625,000
Proceeds From Issuance of Ambac UK Debt										0	12,000	0
Ambac Financial Group, Inc Parent Company Only [Member]
Cash flows from operating activities:
Net income (loss) attributable to common stockholders										(437,000)	(216,000)	186,000
Net income (loss)										(437,000)	(216,000)	186,000
Adjustments to reconcile net income to net cash used in operating activities:
Amortization of bond premium and discount										(6,000)	(6,000)	(7,000)
Share-based compensation										11,000	12,000	12,000
Current income taxes										30,000	15,000	(15,000)
Net realized investment gains										1,000	2,000	2,000
Other, net										(10,000)	(6,000)	0
Net cash used in operating activities										11,000	16,000	32,000
Cash flows from investing activities:
Proceeds from matured bonds										46,000	86,000	230,000
Purchases of bonds										(45,000)	(2,000)	(137,000)
Change in short-term investments										89,000	(125,000)	(123,000)
Payments to Acquire Businesses, Net of Cash Acquired										(74,000)
Net cash provided by investing activities										16,000	(22,000)	(21,000)
Cash flows from financing activities:
AMPS Exchange Cash Paid										0
Cash and Cash Equivalents, Period Increase (Decrease)										(2,000)	(6,000)	11,000
Cash and cash equivalents end of period	$ 7,000				$ 9,000					$ 7,000	9,000	15,000	$ 7,000
Supplemental disclosure of cash flow information
Income Taxes Paid											$ 1,000	$ 4,000