Variable Interest Entities - Effect of Consolidating Investment Vehicles on Consolidated Statement of Operations (Details) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($) fund	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Assets
Other invested assets	$ 260		$ 260			$ 214
Total investments	9,617		9,617			9,838
Premiums receivable, net of commissions payable	1,378		1,378			1,372
Salvage and subrogation recoverable	1,148		1,148			991
Financial guaranty variable interest entities’ assets, at fair value	271		271			296
Assets of CIVs	4,371		4,371			1,913
Other assets	421		421			440
Assets	17,614		17,614			15,334
Liabilities:
Unearned premium reserve	3,716		3,716			3,735
Loss and loss adjustment expense reserve	981		981			1,088
Financial guaranty variable interest entities’ liabilities with recourse, at fair value	281		281			316
FG VIEs’ liabilities without recourse, at fair value	20		20			17
Liabilities of CIVs	3,886		3,886			1,590
Liabilities	11,218		11,218			8,629
Redeemable noncontrolling interests in consolidated investment vehicles	21		21			21
Shareholders’ equity
Retained earnings	5,924		5,924			6,143
Accumulated other comprehensive income	374		374			498
Total shareholders’ equity attributable to Assured Guaranty Ltd.	6,300		6,300			6,643
Nonredeemable noncontrolling interests	75		75			41
Total shareholders’ equity	6,375	$ 6,596	6,375	$ 6,596	$ 6,552	6,684	$ 6,489	$ 6,645
Total liabilities, redeemable noncontrolling interests and shareholders’ equity	17,614		17,614			15,334
Loss on extinguishment of debt	175	$ 0	175	$ 0
Variable Interest Entity, Primary Beneficiary | Financial Guaranty Variable Interest Entities And Consolidated Investment Vehicles
Assets
Fixed maturity securities and short-term investments	(50)		(50)			(32)
Equity method investments	(363)		(363)			(254)
Other invested assets	0		0			(2)
Total investments	(413)		(413)			(288)
Premiums receivable, net of commissions payable	(6)		(6)			(6)
Salvage and subrogation recoverable	(15)		(15)			(9)
Financial guaranty variable interest entities’ assets, at fair value	271		271			296
Assets of CIVs	4,371		4,371			1,913
Other assets	0		0			(3)
Assets	4,208		4,208			1,903
Liabilities:
Unearned premium reserve	(36)		(36)			(38)
Loss and loss adjustment expense reserve	(34)		(34)			(41)
Financial guaranty variable interest entities’ liabilities with recourse, at fair value	281		281			316
FG VIEs’ liabilities without recourse, at fair value	20		20			17
Liabilities of CIVs	3,886		3,886			1,590
Liabilities	4,117		4,117			1,844
Redeemable noncontrolling interests in consolidated investment vehicles	21		21			21
Shareholders’ equity
Retained earnings	22		22			22
Accumulated other comprehensive income	(27)		(27)			(25)
Total shareholders’ equity attributable to Assured Guaranty Ltd.	(5)		(5)			(3)
Nonredeemable noncontrolling interests	75		75			41
Total shareholders’ equity	70		70			38
Total liabilities, redeemable noncontrolling interests and shareholders’ equity	4,208		$ 4,208			$ 1,903
Variable Interest Entity, Primary Beneficiary | Consolidated Investment Vehicles
Shareholders’ equity
Loss on extinguishment of debt	$ 2
Number of financial assets extinguished | fund	1