Variable Interest Entities - Effect of Consolidating Investment Vehicles on Consolidated Statement of Operations (Details) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Variable Interest Entity [Line Items]
Total investment portfolio	$ 9,573		$ 10,240
Premiums receivable, net of commissions payable	1,294		1,286
Salvage and subrogation recoverable	795		747
Financial guaranty variable interest entities’ assets, at fair value	318		442
Assets of consolidated investment vehicles	1,669		572
Other assets (includes $160 and $135 measured at fair value)	513		543
Assets	14,780		14,326
Unearned premium reserve	3,742		3,736
Loss and loss adjustment expense reserve	1,076		1,050
Deferred tax liabilities	45		17
Financial guaranty variable interest entities’ liabilities with recourse, at fair value	332		367
FG VIEs’ liabilities without recourse, at fair value	20		102
Liabilities of consolidated investment vehicles	1,236		482
Other liabilities	480		511
Liabilities	8,271		7,674
Redeemable noncontrolling interests in consolidated investment vehicles	20		7
Retained earnings	6,109		6,295
Accumulated other comprehensive income	333		342
Total shareholders’ equity attributable to Assured Guaranty Ltd.	6,444		6,639
Nonredeemable noncontrolling interests	45		6
Total shareholders’ equity	6,489	$ 6,265	6,645	$ 6,722	$ 6,669	$ 6,555
Total liabilities, redeemable noncontrolling interests and shareholders’ equity	14,780		14,326
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Assets	318		442
Liabilities	352		469
Variable Interest Entity, Primary Beneficiary [Member] | Consolidated Investment Vehicles [Member]
Variable Interest Entity [Line Items]
Fixed maturity securities and short-term investments	(35)		(39)
Equity method investments	(367)		(77)
Total investment portfolio	(402)		(116)
Premiums receivable, net of commissions payable	(6)		(7)
Salvage and subrogation recoverable	(9)		(8)
Financial guaranty variable interest entities’ assets, at fair value	318		442
Assets of consolidated investment vehicles	1,669		572
Other assets (includes $160 and $135 measured at fair value)	(1)		0
Assets	1,569		883
Unearned premium reserve	(41)		(39)
Loss and loss adjustment expense reserve	(46)		(41)
Financial guaranty variable interest entities’ liabilities with recourse, at fair value	332		367
FG VIEs’ liabilities without recourse, at fair value	20		102
Liabilities of consolidated investment vehicles	1,236		482
Other liabilities	1		0
Liabilities	1,502		871
Redeemable noncontrolling interests in consolidated investment vehicles	20		7
Retained earnings	30		34
Accumulated other comprehensive income	(28)		(35)
Total shareholders’ equity attributable to Assured Guaranty Ltd.	2		(1)
Nonredeemable noncontrolling interests	45		6
Total shareholders’ equity	47		5
Total liabilities, redeemable noncontrolling interests and shareholders’ equity	$ 1,569		$ 883