Cover Page - shares	9 Months Ended
	Sep. 30, 2020	Nov. 03, 2020
Entity Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2020
Document Transition Report	false
Entity File Number	001-32141
Entity Registrant Name	ASSURED GUARANTY LTD
Entity Incorporation, State or Country Code	D0
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
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		80,806,180
Entity Central Index Key	0001273813
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Common Shares | New York Stock Exchange
Entity Information [Line Items]
Title of 12(b) Security	Common Shares
Trading Symbol	AGO
Security Exchange Name	NYSE
Assured Guaranty Municipal Holdings Inc. 6-7/8% $100,000,000 Quarterly Interest Bonds due 2101 (and the related guarantee of Registrant) | New York Stock Exchange
Entity Information [Line Items]
Title of 12(b) Security	Assured Guaranty Municipal Holdings Inc. 6-7/8% $100,000,000 Quarterly Interest Bonds due 2101 (and the related guarantee of Registrant)
Trading Symbol	AGO PRB
Security Exchange Name	NYSE
Assured Guaranty Municipal Holdings Inc. 6.25% $230,000,000 Quarterly Interest Bonds due 2102 (and the related guarantee of Registrant) | New York Stock Exchange
Entity Information [Line Items]
Title of 12(b) Security	Assured Guaranty Municipal Holdings Inc. 6.25% $230,000,000 Quarterly Interest Bonds due 2102 (and the related guarantee of Registrant)
Trading Symbol	AGO PRE
Security Exchange Name	NYSE
Assured Guaranty Municipal Holdings Inc. 5.60% $100,000,000 Quarterly Interest Bonds due 2103 (and the related guarantee of Registrant) | New York Stock Exchange
Entity Information [Line Items]
Title of 12(b) Security	Assured Guaranty Municipal Holdings Inc. 5.60% $100,000,000 Quarterly Interest Bonds due 2103 (and the related guarantee of Registrant)
Trading Symbol	AGO PRF
Security Exchange Name	NYSE
Assured Guaranty US Holdings Inc. 5.000% $500,000,000 Senior Notes due 2024 (and the related guarantee of Registrant) | New York Stock Exchange
Entity Information [Line Items]
Title of 12(b) Security	Assured Guaranty US Holdings Inc. 5.000% $500,000,000 Senior Notes due 2024 (and the related guarantee of Registrant)
Trading Symbol	AGO 24
Security Exchange Name	NYSE