Consolidated Balance Sheets - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Assets:
Fixed maturity securities, at fair value	$ 2,858
Total investments (net of allowance for credit losses of $0 and $0)	3,458	$ 3,544
Cash and cash equivalents	23	20
Restricted Cash	16	13
Premiums Receivable, Net	356	370
Reinsurance Recoverables, Including Reinsurance Premium Paid	33	33
Deferred ceded premium	74	70
Subrogation recoverable	2,076	2,156
Derivative assets	74	93
Intangible assets	391	409
Other assets	115	114
Total assets	12,840	13,220
Liabilities:
Unearned premiums	438	456
Loss and loss expense reserves	1,662	1,759
Ceded premiums payable	24	27
Long-term debt	2,661	2,739
Accrued interest payable	516	517
Derivative Liability	86	114
Other liabilities	123	106
Total liabilities	11,697	12,074
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	246	242
Accumulated other comprehensive income	61	79
Retained earnings	761	759
Treasury stock, shares at cost: 0 and 8,202	(5)	(1)
Total Ambac Financial Group, Inc. stockholders’ equity	1,063	1,080
Noncontrolling interest	60	60
Total stockholders’ equity	1,123	1,140
Total liabilities and stockholders’ equity	12,840	13,220
Deferred Income Tax Liabilities, Net	20	24
Taxes Payable, Current	2	6
Available-for-sale Securities, Amortized Cost Basis	2,739	2,807
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Premium Receivable, Allowance for Credit Loss	13	17
Debt Securities, Available-for-sale, Amortized Cost	2,739
Xchange
Liabilities:
Redeemable Noncontrolling Interest, Equity, Redemption Value	20
Redeemable Noncontrolling Interest, Equity, Carrying Amount		7
Short-term Investments [Member]
Assets:
Fixed maturity securities, at fair value	397	492
Stockholders’ equity:
Available-for-sale Securities, Amortized Cost Basis	398	492
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Fixed Income Securities [Member]
Assets:
Fixed maturity securities, at fair value	2,341	2,317
Stockholders’ equity:
Available-for-sale Securities, Amortized Cost Basis	2,221	2,175
Available-for-sale Securities	2,341	2,317
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Short-term Investments [Member]
Stockholders’ equity:
Available-for-sale Securities	397	492
Other Investments [Member]
Stockholders’ equity:
Available-for-sale Securities	600	595
Collateral Pledged [Member] | Fixed Income Securities [Member]
Assets:
Fixed maturity securities, at fair value	120	140
Stockholders’ equity:
Debt Securities, Available-for-sale, Amortized Cost	15	15
Collateral Pledged [Member] | Fixed Income Securities [Member] | Short-term Investments [Member]
Assets:
Fixed maturity securities, at fair value	105	125
Stockholders’ equity:
Available-for-sale Securities, Amortized Cost Basis	105	125
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Collateral Pledged [Member] | Short-term Investments
Stockholders’ equity:
Available-for-sale Securities, Amortized Cost Basis	105	125
Collateralized Securities | Fixed Income Securities [Member]
Stockholders’ equity:
Available-for-sale Securities	15	15
Collateralized Securities | Short-term Investments [Member]
Stockholders’ equity:
Available-for-sale Securities	$ 105	$ 125