| Cover Page Cover Page | Oct. 01, 2019 |
| Cover page. | |
| Document Type | 8-K |
| Document Period End Date | Oct. 1, 2019 |
| Entity Registrant Name | ASSURED GUARANTY LTD. |
| Entity Incorporation, State or Country Code | D0 |
| Entity File Number | 001-32141 |
| Entity Tax Identification Number | 98-0429991 |
| Entity Address, Address Line One | 30 Woodbourne Avenue |
| Entity Address, City or Town | Hamilton |
| Entity Address, Postal Zip Code | HM 08 |
| Entity Address, Country | BM |
| City Area Code | 441 |
| Local Phone Number | 279-5700 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Shares |
| Trading Symbol | AGO |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
| Amendment Flag | false |
| Entity Central Index Key | 0001273813 |
