Consolidated Balance Sheets - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2023	Dec. 31, 2022	Dec. 31, 2021
Assets:
Fixed maturity securities, at fair value	$ 2,004	$ 2,004
Total investments (net of allowance for credit losses of $2 and $0)	2,492	2,492	$ 2,593
Restricted Cash	13	13	14
Premiums Receivable, Net	278	278	269
Premium Receivable, Allowance for Credit Loss	4	4	5
Reinsurance Recoverables on Unpaid Losses, Allowance	0	0	0
Reinsurance Recoverables, Including Reinsurance Premium Paid	172	172	115
Deferred ceded premium	205	205	124
Subrogation recoverable	179	179	271
Derivative assets	15	15	27
Intangible assets, less accumulated amortization	312	312	326
Goodwill	70	70	61	$ 46
Other assets	94	94	84
Total assets	7,847	7,847	7,973
Liabilities:
Unearned premiums	407	407	372
Loss and loss expense reserves	850	850	805
Ceded premiums payable	95	95	39
Long-term debt	505	505	639
Accrued interest payable	462	462	427
Derivative Liability	22	22	38
Other liabilities	168	168	163
Total liabilities	6,507	6,507	6,647
Redeemable Noncontrolling Interest, Equity, Redemption Value	22	22
Redeemable Noncontrolling Interest, Equity, Carrying Amount	22	22	20	18
Liabilities	6,507	6,507	6,647
Stockholders’ equity:
Preferred stock, par value $0.01 per share; 20,000,000 shares authorized; issued and outstanding shares—none	0	0	0
Common Stock, Value, Issued	0	0	0
Additional paid-in capital	286	286	274
Accumulated other comprehensive income	(262)	(262)	(253)	58
Retained earnings	1,257	1,257	1,245
Treasury stock, shares at cost: 0 and 8,202	(17)	(17)	(15)
Total Ambac Financial Group, Inc. stockholders’ equity	1,265	1,265	1,252
Noncontrolling interest	53	53	53
Total stockholders’ equity	1,318	1,318	1,305	1,098
Total liabilities and stockholders’ equity	7,847	7,847	7,973
Debt Securities, Available-for-sale, Allowance for Credit Loss	2	2	0
Debt Securities, Available-for-sale, Amortized Cost	2,100	2,100	2,041
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	302	302	61	$ 23
Deferred Policy Acquisition Cost	8	8	3
Capitalized Contract Cost, Net	$ 7	$ 7	$ 5
Treasury Stock, Common, Shares	1,471,817	1,471,817	1,685,233
Preferred Stock, Shares Issued	0	0	0
Afg [Member]
Stockholders’ equity:
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 46	$ 46	$ 44
Short-term Investments [Member]
Assets:
Fixed maturity securities, at fair value	421	421	507
Stockholders’ equity:
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0	0
Debt Securities, Available-for-sale, Amortized Cost	421	421	507
Variable Interest Entity, Primary Beneficiary [Member]
Assets:
Fixed maturity securities, at fair value	1,948	1,948	1,967
Restricted Cash	256	256	17
Derivative assets	219	219	239
Other assets	1	1	2
Total assets	3,976	3,976	4,054
Liabilities:
Long-term debt	2,699	2,699	3,107
Derivative Liability	1,038	1,038	1,048
Other Sundry Liabilities, Current	255	255	5
Other liabilities	1	1	5
Total liabilities	3,992	3,992	4,160
Liabilities	3,992	3,992	4,160
Stockholders’ equity:
Financing Receivable, after Allowance for Credit Loss	1,551	1,551	1,829
Gross Investment Income, Operating	1
Derivative and other assets	220	220	241
Fixed Income Securities [Member]
Assets:
Fixed maturity securities, at fair value	1,556	1,556	1,395
Stockholders’ equity:
Debt Securities, Available-for-sale, Allowance for Credit Loss	2	2	0
Debt Securities, Available-for-sale, Amortized Cost	1,653	1,653	1,469
Debt Securities, Available-for-Sale, Excluding Accrued Interest	1,556	1,556	1,395
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,651	1,651	1,469
Short-term Investments [Member]
Assets:
Fixed maturity securities, at fair value	421	421	507
Stockholders’ equity:
Gross Investment Income, Operating	5	16
Other Investments [Member]
Stockholders’ equity:
Other Investment Not Readily Marketable, Fair Value	461	461	568
Gross Investment Income, Operating	6	28
Fixed Income Securities, Trading
Stockholders’ equity:
Gross Investment Income, Operating	0	5
Debt Securities, Available-for-Sale, Excluding Accrued Interest	28	28	59
Collateral Pledged [Member] | Fixed Income Securities [Member]
Assets:
Fixed maturity securities, at fair value	27	27	64
Collateral Pledged [Member] | Fixed Income Securities [Member] | Short-term Investments [Member]
Assets:
Fixed maturity securities, at fair value	27	27	64
Stockholders’ equity:
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0	0
Debt Securities, Available-for-sale, Amortized Cost	27	27	64
Collateral Pledged [Member] | Short-term Investments
Stockholders’ equity:
Debt Securities, Available-for-sale, Amortized Cost	27	27	64
Collateralized Securities | Short-term Investments [Member]
Assets:
Fixed maturity securities, at fair value	$ 27	$ 27	$ 64