Variable Interest Entities - Effect of Consolidating Investment Vehicles on Consolidated Statement of Operations (Details) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Variable Interest Entity [Line Items]
Total investment portfolio	$ 9,527		$ 10,240
Premiums receivable, net of commissions payable	1,321		1,286
Salvage and subrogation recoverable	961		747
Financial guaranty variable interest entities’ assets, at fair value	314		442
Assets of consolidated investment vehicles	1,539		572
Assets	14,695		14,326
Unearned premium reserve	3,762		3,736
Loss and loss adjustment expense reserve	982		1,050
Financial guaranty variable interest entities’ liabilities with recourse, at fair value	336		367
FG VIEs’ liabilities without recourse, at fair value	19		102
Liabilities of consolidated investment vehicles	1,092		482
Liabilities	8,078		7,674
Redeemable noncontrolling interests in consolidated investment vehicles	21		7
Retained earnings	6,143		6,295
Accumulated other comprehensive income	404		342
Total shareholders’ equity attributable to Assured Guaranty Ltd.	6,549		6,639
Nonredeemable noncontrolling interests	47		6
Total shareholders’ equity	6,596	$ 6,489	6,645	$ 6,652	$ 6,722	$ 6,555
Total liabilities, redeemable noncontrolling interests and shareholders’ equity	14,695		14,326
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Assets	314		442
Liabilities	355		469
Variable Interest Entity, Primary Beneficiary | Consolidated Investment Vehicles
Variable Interest Entity [Line Items]
Fixed maturity securities and short-term investments	(33)		(39)
Equity method investments	(379)		(77)
Total investment portfolio	(412)		(116)
Premiums receivable, net of commissions payable	(6)		(7)
Salvage and subrogation recoverable	(9)		(8)
Financial guaranty variable interest entities’ assets, at fair value	314		442
Assets of consolidated investment vehicles	1,539		572
Assets	1,426		883
Unearned premium reserve	(41)		(39)
Loss and loss adjustment expense reserve	(44)		(41)
Financial guaranty variable interest entities’ liabilities with recourse, at fair value	336		367
FG VIEs’ liabilities without recourse, at fair value	19		102
Liabilities of consolidated investment vehicles	1,092		482
Liabilities	1,362		871
Redeemable noncontrolling interests in consolidated investment vehicles	21		7
Retained earnings	27		34
Accumulated other comprehensive income	(31)		(35)
Total shareholders’ equity attributable to Assured Guaranty Ltd.	(4)		(1)
Nonredeemable noncontrolling interests	47		6
Total shareholders’ equity	43		5
Total liabilities, redeemable noncontrolling interests and shareholders’ equity	$ 1,426		$ 883