Consolidated Balance Sheets - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Assets:
Fixed maturity securities, at fair value	$ 2,095
Total investments (net of allowance for credit losses of $2 and $3)	2,682	$ 2,664
Restricted Cash	11	12
Premiums Receivable, Net	299	290
Premium Receivable, Allowance for Credit Loss	3	4
Reinsurance Recoverables on Unpaid Losses, Allowance	0	0
Reinsurance Recoverables, Including Reinsurance Premium Paid	224	195
Deferred ceded premium	217	204
Subrogation recoverable	130	137
Derivative assets	21	26
Intangible assets, less accumulated amortization	293	307
Goodwill	70	70	$ 61
Other assets	129	129
Total assets	8,429	8,428
Liabilities:
Unearned premiums	429	422
Liability for Claims and Claims Adjustment Expense	851	893
Ceded premiums payable	110	90
Long-term debt	512	508
Accrued interest payable	487	475
Derivative Liability	29	35
Other liabilities	259	199
Total liabilities	6,993	6,997
Redeemable Noncontrolling Interest, Equity, Redemption Value	17
Redeemable Noncontrolling Interest, Equity, Carrying Amount	17	17	20
Liabilities	6,993	6,997
Stockholders’ equity:
Preferred stock, par value $0.01 per share; 20,000,000 shares authorized; issued and outstanding shares—none	0	0
Common Stock, Value, Issued	0	0
Additional paid-in capital	291	292
Accumulated other comprehensive income	(175)	(160)	(253)
Retained earnings	1,266	1,246
Treasury stock, shares at cost: 0 and 8,202	(17)	(17)
Total Ambac Financial Group, Inc. stockholders’ equity	1,365	1,362
Noncontrolling interest	53	53
Total stockholders’ equity	1,418	1,415	1,305
Total liabilities and stockholders’ equity	8,429	8,428
Debt Securities, Available-for-sale, Allowance for Credit Loss	2	3
Debt Securities, Available-for-sale, Amortized Cost	2,135	2,197
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	296	274	$ 61
Deferred Policy Acquisition Cost	12	11
Capitalized Contract Cost, Net	$ 7	$ 6
Treasury Stock, Common, Shares	1,434,172	1,463,774
Preferred Stock, Shares Issued	0	0
Afg [Member]
Stockholders’ equity:
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 44	$ 28
Short-term Investments [Member]
Assets:
Fixed maturity securities, at fair value	382	426
Stockholders’ equity:
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Amortized Cost	382	426
Variable Interest Entity, Primary Beneficiary [Member]
Assets:
Fixed maturity securities, at fair value	2,162	2,167
Restricted Cash	252	246
Derivative assets	221	226
Other assets	92	92
Total assets	4,330	4,394
Liabilities:
Long-term debt	2,925	2,967
Derivative Liability	1,170	1,197
Other Sundry Liabilities, Current	245	240
Other liabilities	6	5
Total liabilities	4,340	4,404
Liabilities	4,340	4,404
Stockholders’ equity:
Financing Receivable, after Allowance for Credit Loss	1,604	1,663
Gross Investment Income, Operating	3
Derivative and other assets	313	318
Fixed Income Securities [Member]
Assets:
Fixed maturity securities, at fair value	1,687	1,710
Stockholders’ equity:
Debt Securities, Available-for-sale, Allowance for Credit Loss	2	3
Debt Securities, Available-for-sale, Amortized Cost	1,726	1,744
Debt Securities, Available-for-Sale, Excluding Accrued Interest	1,687	1,710
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,726	1,744
Short-term Investments [Member]
Assets:
Fixed maturity securities, at fair value	382	426
Stockholders’ equity:
Gross Investment Income, Operating	5
Other Investments [Member]
Stockholders’ equity:
Other Investment Not Readily Marketable, Fair Value	558	475
Gross Investment Income, Operating	13
Fixed Income Securities, Trading
Stockholders’ equity:
Gross Investment Income, Operating	2
Debt Securities, Available-for-Sale, Excluding Accrued Interest	29	27
Collateral Pledged [Member] | Fixed Income Securities [Member]
Assets:
Fixed maturity securities, at fair value	26	27
Stockholders’ equity:
Debt Securities, Available-for-sale, Amortized Cost	27	0
Collateral Pledged [Member] | Fixed Income Securities [Member] | Short-term Investments [Member]
Assets:
Fixed maturity securities, at fair value	0	27
Stockholders’ equity:
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Amortized Cost	0	27
Collateral Pledged [Member] | Short-term Investments
Stockholders’ equity:
Debt Securities, Available-for-sale, Amortized Cost	0	27
Collateralized Securities | Fixed Income Securities [Member]
Stockholders’ equity:
Debt Securities, Available-for-Sale, Excluding Accrued Interest	26	0
Collateralized Securities | Short-term Investments [Member]
Assets:
Fixed maturity securities, at fair value	$ 0	$ 27