Variable Interest Entities - Narrative (Details)	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Feb. 28, 2017 USD ($)	Jun. 30, 2020 USD ($) policy Entity	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) policy Entity	Jun. 30, 2019 USD ($) Entity	Dec. 31, 2019 USD ($) policy Entity
Variable Interest Entity [Line Items]
Assets		$ 14,780,000,000		$ 14,780,000,000		$ 14,326,000,000
Total shareholders’ equity attributable to Assured Guaranty Ltd.		6,444,000,000		6,444,000,000		6,639,000,000
Net fair value gains and losses on FG VIEs are expected to reverse to zero at maturity of the VIE debt		0		0
Fair value gains (losses) on FG VIEs		1,000,000	$ 33,000,000	(8,000,000)	$ 38,000,000
Variable Interest Entity, Other Consolidated [Abstract]
Other consolidated VIE assets		95,000,000		95,000,000		91,000,000
Other consolidated VIE liabilities		$ 13,000,000		$ 13,000,000		$ 12,000,000
Number of Policies Monitored | policy		18,000		18,000
Number of Policies Monitored, Not Within the Scope of ASC 810 | policy		16,000		16,000
Number of Policies that Contain Provisions for Consolidation | policy		88		88		90
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Assets		$ 318,000,000		$ 318,000,000		$ 442,000,000
Fair value gains (losses) on FG VIEs		$ 1,000,000	33,000,000	$ (8,000,000)	$ 38,000,000
Variable Interest Entity, Other Consolidated [Abstract]
Total number of entities consolidated | Entity		27		27		27
FG VIE matured in the period, number | Entity				2
FG VIEs consolidated in the period | Entity				2
FG VIEs Deconsolidated in the period | Entity					2
Residential Mortgage Backed Securities and Other Insurance Products [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Change in the instrument specific credit risk of the VIEs' assets		$ 13,000,000	$ (29,000,000)	$ 16,000,000	$ (35,000,000)
Future Equity Investments [Member] | Maximum [Member]
Variable Interest Entity [Line Items]
Long-term purchase commitment, amount	$ 100,000,000
Variable Interest Entity, Other Consolidated [Abstract]
Investment Company, Committed Capital				12,000,000
Future Equity Investments [Member] | AGL Subsidiaries [Member] | Assured Investment Management Funds [Member] | Maximum [Member]
Variable Interest Entity [Line Items]
Long-term purchase commitment, amount				500,000,000		$ 500,000,000
Equity Method Investments [Member] | AGL Subsidiaries [Member] | Assured Investment Management Funds [Member] | Maximum [Member]
Variable Interest Entity [Line Items]
Long-term purchase commitment, amount				354,000,000		79,000,000
Variable Interest Entity, Other Consolidated [Abstract]
Long-term purchase commitment fair value		367,000,000		367,000,000		77,000,000
Consolidated Investment Vehicles [Member]
Variable Interest Entity [Line Items]
Fair value gains (losses) on FG VIEs		1,000,000		(8,000,000)
Consolidated Investment Vehicles [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Assets		1,569,000,000		1,569,000,000		883,000,000
Total shareholders’ equity attributable to Assured Guaranty Ltd.		$ 2,000,000		$ 2,000,000		$ (1,000,000)