Fair Value Measurements - Summary of Carrying Amount and Fair Value of Ambac's Financial Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	$ 3,048
Financial assets:
Derivative assets	95	$ 75
Other assets	2	3
Long-term Debt	2,737	2,822
Financial liabilities:
Derivative liabilities	126	90
Short-term Investments [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	586	653
Total Fair Value [Member]
Financial assets:
Cash	48	79
Reported Value Measurement [Member]
Financial assets:
Other investments	60	136
Cash	48	79
Loans	3	10
Other assets	2	3
Total financial assets	9,696	10,242
Financial liabilities:
Liabilities for net financial guarantees written	(744)	(863)
Long-term debt	3,236	3,262
Total financial liabilities	8,558	8,699
Reported Value Measurement [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	586	653
Reported Value Measurement [Member] | Other Debt Obligations [Member]
Financial assets:
Other investments	502	478
Reported Value Measurement [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	125	89
Reported Value Measurement [Member] | Other Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	1
Other Assets [Member] | Reported Value Measurement [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	95	75
Level 1 [Member]
Financial assets:
Cash	39	70
Total financial assets	965	1,091
Financial liabilities:
Liabilities for net financial guarantees written	0	0
Long-term debt	0	0
Total financial liabilities	0	0
Level 1 [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	509	598
Level 1 [Member] | Equity Method Investments [Member]
Financial assets:
Other assets	0	0
Level 1 [Member] | Loans Receivable [Member]
Financial assets:
Loans	0	0
Level 1 [Member] | Other Debt Obligations [Member]
Financial assets:
Other investments	62	136
Level 1 [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	0	0
Level 1 [Member] | Other Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	0
Level 1 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	0	0
Level 2 [Member]
Financial assets:
Cash	9	9
Total financial assets	2,312	2,593
Financial liabilities:
Liabilities for net financial guarantees written	0	0
Long-term debt	2,679	2,829
Total financial liabilities	8,601	8,983
Level 2 [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	77	55
Level 2 [Member] | Equity Method Investments [Member]
Financial assets:
Other assets	0	0
Level 2 [Member] | Loans Receivable [Member]
Financial assets:
Loans	0	0
Level 2 [Member] | Other Debt Obligations [Member]
Financial assets:
Other investments	0	0
Level 2 [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	125	89
Level 2 [Member] | Other Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	0
Level 2 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	9	8
Level 3 [Member]
Financial assets:
Cash	0	0
Total financial assets	6,013	6,281
Financial liabilities:
Liabilities for net financial guarantees written	436	284
Long-term debt	377	445
Total financial liabilities	968	889
Level 3 [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	0	0
Level 3 [Member] | Equity Method Investments [Member]
Financial assets:
Other assets	2	3
Level 3 [Member] | Loans Receivable [Member]
Financial assets:
Loans	3	13
Level 3 [Member] | Other Debt Obligations [Member]
Financial assets:
Other investments	35	61
Level 3 [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	0	0
Level 3 [Member] | Other Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	1
Level 3 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	86	67
Commercial Mortgage Backed Securities [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value		50
Municipal Bonds [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	351	215
Corporate Debt Securities [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	1,091	1,430
Debt Security, Government, Non-US [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	77	44
US Government Debt Securities [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	122	156
Residential Mortgage-Backed Securities [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	296	248
Collateralized Debt Obligations [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	73	146
Asset-backed Securities [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	300	287
Fair Value, Recurring [Member]
Financial assets:
Total financial assets	9,682	10,260
Financial liabilities:
Liabilities for net financial guarantees written	436	284
Long-term debt	3,056	3,274
Total financial liabilities	9,569	9,872
Fair Value, Recurring [Member] | Commercial Mortgage Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value		50
Fair Value, Recurring [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	586	653
Fair Value, Recurring [Member] | Equity Method Investments [Member]
Financial assets:
Other assets	2	3
Fair Value, Recurring [Member] | Loans Receivable [Member]
Financial assets:
Loans	3	13
Fair Value, Recurring [Member] | Other Debt Obligations [Member]
Financial assets:
Other investments	488	493
Fair Value, Recurring [Member] | Asset-backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	300	287
Fair Value, Recurring [Member] | Collateralized Debt Obligations [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	73	146
Fair Value, Recurring [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	296	248
Fair Value, Recurring [Member] | US Government Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	122	156
Fair Value, Recurring [Member] | Debt Security, Government, Non-US [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	77	44
Fair Value, Recurring [Member] | Corporate Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	1,091	1,430
Fair Value, Recurring [Member] | Municipal Bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	351	215
Fair Value, Recurring [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	125	89
Fair Value, Recurring [Member] | Other Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	1
Fair Value, Recurring [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	95	75
Fair Value, Recurring [Member] | Level 1 [Member] | Commercial Mortgage Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value		0
Fair Value, Recurring [Member] | Level 1 [Member] | Asset-backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	0	0
Fair Value, Recurring [Member] | Level 1 [Member] | Collateralized Debt Obligations [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	0	0
Fair Value, Recurring [Member] | Level 1 [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	0	0
Fair Value, Recurring [Member] | Level 1 [Member] | US Government Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	122	156
Fair Value, Recurring [Member] | Level 1 [Member] | Debt Security, Government, Non-US [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	77	44
Fair Value, Recurring [Member] | Level 1 [Member] | Corporate Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	2	0
Fair Value, Recurring [Member] | Level 1 [Member] | Municipal Bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	0	0
Fair Value, Recurring [Member] | Level 2 [Member] | Commercial Mortgage Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value		50
Fair Value, Recurring [Member] | Level 2 [Member] | Asset-backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	225	215
Fair Value, Recurring [Member] | Level 2 [Member] | Collateralized Debt Obligations [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	73	146
Fair Value, Recurring [Member] | Level 2 [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	296	248
Fair Value, Recurring [Member] | Level 2 [Member] | US Government Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	0	0
Fair Value, Recurring [Member] | Level 2 [Member] | Debt Security, Government, Non-US [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	0	0
Fair Value, Recurring [Member] | Level 2 [Member] | Corporate Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	1,089	1,430
Fair Value, Recurring [Member] | Level 2 [Member] | Municipal Bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	351	215
Fair Value, Recurring [Member] | Level 3 [Member] | Commercial Mortgage Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value		0
Fair Value, Recurring [Member] | Level 3 [Member] | Asset-backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	75	72
Fair Value, Recurring [Member] | Level 3 [Member] | Collateralized Debt Obligations [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	0	0
Fair Value, Recurring [Member] | Level 3 [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	0	0
Fair Value, Recurring [Member] | Level 3 [Member] | US Government Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	0	0
Fair Value, Recurring [Member] | Level 3 [Member] | Debt Security, Government, Non-US [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	0	0
Fair Value, Recurring [Member] | Level 3 [Member] | Corporate Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	0	0
Fair Value, Recurring [Member] | Level 3 [Member] | Municipal Bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	0	0
Collateral Pledged [Member] | Short-term Investments [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value		85
Collateral Pledged [Member] | Short-term Investments [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	152	85
Collateral Pledged [Member] | Fair Value, Recurring [Member] | Short-term Investments [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	152	85
Collateral Pledged [Member] | Fair Value, Recurring [Member] | Level 1 [Member] | Short-term Investments [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	152	85
Collateral Pledged [Member] | Fair Value, Recurring [Member] | Level 2 [Member] | Short-term Investments [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	0	0
Collateral Pledged [Member] | Fair Value, Recurring [Member] | Level 3 [Member] | Short-term Investments [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	0	0
Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	3,160	3,121
Financial assets:
Loans	2,783	3,108
Derivative assets	53	52
Long-term Debt	4,169	4,554
Financial liabilities:
Derivative liabilities	1,771	1,657
Long-term debt	4,002	4,351
Variable Interest Entity, Primary Beneficiary [Member] | Corporate Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	3,029	2,957
Variable Interest Entity, Primary Beneficiary [Member] | Municipal Bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	131	164
Variable Interest Entity, Primary Beneficiary [Member] | Reported Value Measurement [Member]
Financial assets:
Restricted Cash and Cash Equivalents, Current	2	2
Loans	2,783	3,108
Financial liabilities:
Long-term debt	4,169	4,554
Variable Interest Entity, Primary Beneficiary [Member] | Reported Value Measurement [Member] | Corporate Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	3,029	2,957
Variable Interest Entity, Primary Beneficiary [Member] | Reported Value Measurement [Member] | Municipal Bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	131	164
Variable Interest Entity, Primary Beneficiary [Member] | Reported Value Measurement [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	1,771	1,657
Variable Interest Entity, Primary Beneficiary [Member] | Reported Value Measurement [Member] | Currency Swaps [Member]
Financial assets:
Derivative assets	53	52
Variable Interest Entity, Primary Beneficiary [Member] | Level 1 [Member]
Financial assets:
Restricted Cash and Cash Equivalents, Current	2	2
Loans	0	0
Financial liabilities:
Long-term debt	0	0
Variable Interest Entity, Primary Beneficiary [Member] | Level 1 [Member] | Corporate Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	0	0
Variable Interest Entity, Primary Beneficiary [Member] | Level 1 [Member] | Municipal Bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	0	0
Variable Interest Entity, Primary Beneficiary [Member] | Level 1 [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	0	0
Variable Interest Entity, Primary Beneficiary [Member] | Level 1 [Member] | Currency Swaps [Member]
Financial assets:
Derivative assets	0	0
Variable Interest Entity, Primary Beneficiary [Member] | Level 2 [Member]
Financial assets:
Restricted Cash and Cash Equivalents, Current	0	0
Loans	0	0
Financial liabilities:
Long-term debt	4,026	4,408
Variable Interest Entity, Primary Beneficiary [Member] | Level 2 [Member] | Corporate Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	0	0
Variable Interest Entity, Primary Beneficiary [Member] | Level 2 [Member] | Municipal Bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	131	164
Variable Interest Entity, Primary Beneficiary [Member] | Level 2 [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	1,771	1,657
Variable Interest Entity, Primary Beneficiary [Member] | Level 2 [Member] | Currency Swaps [Member]
Financial assets:
Derivative assets	53	52
Variable Interest Entity, Primary Beneficiary [Member] | Level 3 [Member]
Financial assets:
Restricted Cash and Cash Equivalents, Current	0	0
Loans	2,783	3,108
Financial liabilities:
Long-term debt	153	159
Variable Interest Entity, Primary Beneficiary [Member] | Level 3 [Member] | Corporate Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	3,029	2,957
Variable Interest Entity, Primary Beneficiary [Member] | Level 3 [Member] | Municipal Bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	0	0
Variable Interest Entity, Primary Beneficiary [Member] | Level 3 [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	0	0
Variable Interest Entity, Primary Beneficiary [Member] | Level 3 [Member] | Currency Swaps [Member]
Financial assets:
Derivative assets	0	0
Variable Interest Entity, Primary Beneficiary [Member] | Fair Value, Recurring [Member]
Financial assets:
Restricted Cash and Cash Equivalents, Current	2	2
Loans	2,783	3,108
Financial liabilities:
Long-term debt	4,179	4,567
Variable Interest Entity, Primary Beneficiary [Member] | Fair Value, Recurring [Member] | Corporate Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	3,029	2,957
Variable Interest Entity, Primary Beneficiary [Member] | Fair Value, Recurring [Member] | Municipal Bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value	131	164
Variable Interest Entity, Primary Beneficiary [Member] | Fair Value, Recurring [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	1,771	1,657
Variable Interest Entity, Primary Beneficiary [Member] | Fair Value, Recurring [Member] | Currency Swaps [Member]
Financial assets:
Derivative assets	$ 53	$ 52