Variable Interest Entities - Effect of Consolidating Investment Vehicles on Consolidated Statement of Operations (Details) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Variable Interest Entity [Line Items]
Other invested assets	$ 211	$ 214
Total investment portfolio	9,599	9,838
Premiums receivable, net of commissions payable	1,359	1,372
Salvage and subrogation recoverable	977	991
Financial guaranty variable interest entities’ assets, at fair value	281	296
Assets of CIVs	2,956	1,913
Other assets	443	440
Assets	16,018	15,334
Unearned premium reserve	3,718	3,735
Loss and loss adjustment expense reserve	1,058	1,088
Financial guaranty variable interest entities’ liabilities with recourse, at fair value	301	316
FG VIEs’ liabilities without recourse, at fair value	17	17
Liabilities of CIVs	2,573	1,590
Liabilities	9,525	8,629
Redeemable noncontrolling interests in consolidated investment vehicles	21	21
Retained earnings	6,055	6,143
Accumulated other comprehensive income	373	498
Total shareholders’ equity attributable to Assured Guaranty Ltd.	6,430	6,643
Nonredeemable noncontrolling interests	42	41
Total shareholders’ equity	6,472	6,684	$ 6,265	$ 6,645
Total liabilities, redeemable noncontrolling interests and shareholders’ equity	16,018	15,334
Variable Interest Entity, Primary Beneficiary | Financial Guaranty Variable Interest Entities And Consolidated Investment Vehicles
Variable Interest Entity [Line Items]
Fixed maturity securities and short-term investments	(51)	(32)
Equity method investments	(297)	(254)
Other invested assets	0	(2)
Total investment portfolio	(348)	(288)
Premiums receivable, net of commissions payable	(6)	(6)
Salvage and subrogation recoverable	(10)	(9)
Financial guaranty variable interest entities’ assets, at fair value	281	296
Assets of CIVs	2,956	1,913
Other assets	1	(3)
Assets	2,874	1,903
Unearned premium reserve	(37)	(38)
Loss and loss adjustment expense reserve	(39)	(41)
Financial guaranty variable interest entities’ liabilities with recourse, at fair value	301	316
FG VIEs’ liabilities without recourse, at fair value	17	17
Liabilities of CIVs	2,573	1,590
Liabilities	2,815	1,844
Redeemable noncontrolling interests in consolidated investment vehicles	21	21
Retained earnings	22	22
Accumulated other comprehensive income	(26)	(25)
Total shareholders’ equity attributable to Assured Guaranty Ltd.	(4)	(3)
Nonredeemable noncontrolling interests	42	41
Total shareholders’ equity	38	38
Total liabilities, redeemable noncontrolling interests and shareholders’ equity	$ 2,874	$ 1,903