| Cover Document | Dec. 31, 2020 |
| Entity Information [Line Items] | |
| Entity Central Index Key | 0000874501 |
| Entity Emerging Growth Company | false |
| Written Communications | false |
| City Area Code | (212) |
| Entity Address, Address Line One | One World Trade Center |
| Entity Address, City or Town | New York |
| Entity Address, State or Province | NY |
| Entity Incorporation, State or Country Code | DE |
| Entity Registrant Name | Ambac Financial Group, Inc. |
| Document Period End Date | Dec. 31, 2020 |
| Document Type | 8-K |
| Entity File Number | 1-10777 |
| Entity Tax Identification Number | 13-3621676 |
| Entity Address, Postal Zip Code | 10007 |
| Local Phone Number | 658-7470 |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Amendment Flag | false |
| Common Stock [Member] | |
| Entity Information [Line Items] | |
| Title of 12(b) Security | Common stock, par value $0.01 per share |
| Trading Symbol | AMBC |
| Security Exchange Name | NYSE |
| Warrant [Member] | |
| Entity Information [Line Items] | |
| Title of 12(b) Security | Warrants |
| Trading Symbol | AMBCW |
| Security Exchange Name | NYSE |
