Schedule II - Condensed Financial Information of Registrant (Parent Company Only) - Condensed Statements of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended			21 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2023
Cash flows from operating activities:
Net income	$ 5.0	$ 522.0	$ (16.0)
Adjustments to reconcile net income loss to net cash used in operating activities:
Accretion (Amortization) of Discounts and Premiums, Investments	(15.0)	(11.0)	(13.0)
Net investment gains (losses), including impairments	22.0	(31.0)	(7.0)
Share-based compensation	17.0	17.0	14.0
Proceeds from Contributions from Affiliates	8.0	6.0	6.0
Other, net	13.0	(196.0)	(56.0)
Net cash provided by operating activities	200.0	1,335.0	(131.0)
Cash flows from investing activities:
Proceeds from matured bonds	74.0	206.0	698.0
Purchases of bonds	(415.0)	(403.0)	(343.0)
Change in short-term investments	118.0	(52.0)	98.0
Proceeds from Sale and Maturity of Other Investments	209.0	166.0	39.0
Payments for (Proceeds from) Other Investing Activities	6.0	9.0	(5.0)
Net cash provided by investing activities	435.0	866.0	776.0
Cash flows from financing activities:
Net Cash Provided by (Used in) Financing Activities	(423.0)	(2,163.0)	(657.0)
Cash and cash equivalents at beginning of period	31.0	17.0
Cash and cash equivalents end of period	16.0	31.0	17.0	$ 16.0
Cash paid during the period for:
Income taxes	11.0	6.0	15.0
Payments to Acquire Businesses, Net of Cash Acquired	7.0	18.0	0.0
Payments for Repurchase of Common Stock	5.0	14.0		19.0
Ambac Financial Group, Inc Parent Company Only [Member]
Cash flows from operating activities:
Net income	4.0	522.0	(17.0)
Adjustments to reconcile net income loss to net cash used in operating activities:
Equity in undistributed net (income) loss of non-debtor subsidiaries	(15.0)	(542.0)	1.0
Accretion (Amortization) of Discounts and Premiums, Investments	0.0	(7.0)	(9.0)
Net investment gains (losses), including impairments	0.0	14.0	5.0
Decrease in current income taxes payable	(2.0)	(1.0)	1.0
Share-based compensation	9.0	12.0	14.0
Decrease in other assets	(5.0)	41.0	(5.0)
Other, net	2.0	1.0	(6.0)
Net cash provided by operating activities	2.0	46.0	(10.0)
Cash flows from investing activities:
Proceeds from matured bonds	0.0	68.0	33.0
Purchases of bonds	(1.0)	(1.0)	(34.0)
Change in short-term investments	20.0	(51.0)	105.0
Proceeds from Sale and Maturity of Other Investments	(3.0)	(4.0)	(8.0)
Net cash provided by investing activities	16.0	12.0	95.0
Capital Contribution to Subsidiaries	(16.0)	(42.0)	(92.0)
Cash flows from financing activities:
Net Cash Provided by (Used in) Financing Activities	(21.0)	(57.0)	(92.0)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect	(2.0)	1.0	(6.0)
Cash and cash equivalents at beginning of period	3.0	1.0	7.0
Cash and cash equivalents end of period	0.0	3.0	1.0	$ 0.0
Cash paid during the period for:
Income taxes	0.0	0.0	$ 0.0
Payments for Repurchase of Common Stock	$ (5.0)	$ (14.0)