Schedule II - Condensed Financial Information of Registrant (Parent Company Only) - Condensed Statements of Cash Flows (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended			18 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	Dec. 31, 2016	Aug. 03, 2018
Condensed Cash Flow Statements, Captions [Line Items]
AMPS Exchange Dollar Value Warrants Issued													$ 8,000
Cash flows from operating activities:
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)
Adjustments to reconcile net income loss to net cash used in operating activities:
Accretion (Amortization) of Discounts and Premiums, Investments									(63,000)	(137,000)	(183,000)
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)
Decrease in current income taxes payable									35,000	(35,000)	(26,000)
Share-based compensation									12,000	12,000	4,000
Other, net									80,000	67,000	22,000
Net cash used in operating activities									(311,000)	(1,543,000)	(221,000)
Cash flows from investing activities:
Proceeds from matured bonds									379,000	432,000	814,000
Purchases of bonds									(959,000)	(528,000)	(2,054,000)
Change in short-term investments									(218,000)	127,000	(127,000)
Proceeds from Sale and Maturity of Other Investments									81,000	159,000	350,000
Payments for (Proceeds from) Other Investing Activities									(2,000)	0	(17,000)
Net cash provided by investing activities									1,000,000	1,588,000	1,163,000
AMPS Proceeds									19,000
Cash flows from financing activities:
Payments for Repurchase of Warrants												$ 8,000
AMPS Exchange Cash Paid									0	(11,000)
Net Cash Provided by (Used in) Financing Activities									(691,000)	(585,000)	(412,000)
Cash and Cash Equivalents, Period Increase (Decrease)									(2,000)	(541,000)	529,000
Cash and cash equivalents at beginning of period				63,000				624,000	63,000	624,000
Cash and cash equivalents end of period	24,000				63,000				24,000	63,000	624,000
Cash paid during the period for:
Income taxes									21,000	35,000	40,000
Ambac Financial Group, Inc Parent Company Only [Member]
Cash flows from operating activities:
Net income (loss)									(216,000)	186,000	(329,000)
Adjustments to reconcile net income loss to net cash used in operating activities:
Equity in undistributed net (income) loss of non-debtor subsidiaries									223,000	(157,000)	371,000
Accretion (Amortization) of Discounts and Premiums, Investments									(6,000)	(7,000)	(17,000)
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
Decrease in current income taxes payable									15,000	(15,000)	(1,000)
Share-based compensation									12,000	12,000	4,000
Decrease in other assets									(8,000)	12,000	(11,000)
Other, net									(6,000)	0	(10,000)
Net cash used in operating activities									16,000	32,000	16,000
Cash flows from investing activities:
Proceeds from matured bonds									86,000	230,000	187,000
Purchases of bonds									(2,000)	(137,000)	(196,000)
Change in short-term investments									(125,000)	(123,000)	(3,000)
Net cash provided by investing activities									(22,000)	(21,000)	(44,000)
Cash flows from financing activities:
AMPS Exchange Cash Paid									0
Cash and Cash Equivalents, Period Increase (Decrease)									(6,000)	11,000	(28,000)
Cash and cash equivalents at beginning of period				$ 15,000				$ 4,000	15,000	4,000	32,000
Cash and cash equivalents end of period	$ 9,000				$ 15,000				9,000	15,000	4,000	$ 32,000
Cash paid during the period for:
Income taxes									1,000	4,000	1,000
Parent [Member]
Cash flows from investing activities:
Proceeds from Sale and Maturity of Other Investments									0	25,000	(35,000)
Payments for (Proceeds from) Other Investing Activities									0	(5,000)	3,000
AMPS Proceeds									$ 19,000
Cash flows from financing activities:
AMPS Exchange Cash Paid										$ (11,000)	$ 0