Variable Interest Entities - Effect of Consolidating Investment Vehicles on Financial Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Balance Sheet Related Disclosures [Abstract]
Investments	$ 10,240				$ 10,873				$ 10,240	$ 10,873
Premiums receivable, net of commissions payable	1,286				904				1,286	904
Salvage and subrogation recoverable	747				490				747	490
Financial guaranty variable interest entities’ assets, at fair value	442				569				442	569
Assets of consolidated investment vehicles (includes $558 measured at fair value)	572				0				572	0
Assets	14,326				13,603				14,326	13,603
Unearned premium reserve	3,736				3,512				3,736	3,512
Loss and loss adjustment expense reserve	1,050				1,177				1,050	1,177
Financial guaranty variable interest entities’ liabilities with recourse, at fair value	367				517				367	517
Financial guaranty variable interest entities’ liabilities without recourse, at fair value	102				102				102	102
Liabilities of consolidated investment vehicles (includes $481 measured at fair value)	482				0				482	0
Liabilities	7,674				7,048				7,674	7,048
Redeemable noncontrolling interests in consolidated investment vehicles	7				0				7	0
Retained earnings	6,295				6,374				6,295	6,374
Accumulated other comprehensive income, net of tax of $71 and $38	342				93				342	93
Total shareholders' equity attributable to AGL	6,639				6,555				6,639	6,555
Nonredeemable noncontrolling interests	6				0				6	0
Total shareholders’ equity	6,645				6,555				6,645	6,555	$ 6,839	$ 6,504
Liabilities and Equity	14,326				13,603				14,326	13,603
Income Statement Related Disclosures [Abstract]
Net earned premiums	123	$ 123	$ 112	$ 118	125	$ 142	$ 136	$ 145	476	548	690
Net investment income	82	88	110	98	98	99	98	100	378	395	417
Fair value gains (losses) on financial guaranty variable interest entities									42	14	30
Other income (loss)	(8)	(9)	24	(3)	11	22	(8)	(8)	4	17	5
Loss and LAE	18	30	(1)	46	21	17	44	(18)	93	64	388
Equity in net earnings of investees	1	0	1	2	1	(1)	1	0	4	1	0
Income (loss) before income taxes and equity in net earnings of investees	137	86	181	56	100	176	86	217	460	579	991
Provision (benefit) for income taxes	2	17	40	4	13	14	12	20	63	59	261
Net income (loss)	136	69	142	54	88	161	75	197	401	521	730
Less: noncontrolling interests	(1)	0	0	0	0	0	0	0	(1)	0	0
Net income (loss) attributable to Assured Guaranty Ltd.	137	$ 69	$ 142	$ 54	88	$ 161	$ 75	$ 197	402	521	730
Effect on OCI									4	2	0
Supplemental cash flow information
Effect on cash flows from operating activities									(509)	462	433
Effect on cash flows from investing activities									1,169	297	345
Effect on cash flows from financing activities									(584)	(795)	(766)
Increase (decrease) in cash and restricted cash									79	(40)	17
Variable Interest Entity, Primary Beneficiary [Member]
Balance Sheet Related Disclosures [Abstract]
Fixed-maturity securities, available-for-sale, at fair value (amortized cost of $8,371 and $9,884)	(39)				(38)				(39)	(38)
Equity Method Investments	(77)				0				(77)	0
Investments	(116)				(38)				(116)	(38)
Premiums receivable, net of commissions payable	(7)				(9)				(7)	(9)
Salvage and subrogation recoverable	(8)				(1)				(8)	(1)
Financial guaranty variable interest entities’ assets, at fair value	442				569				442	569
Assets of consolidated investment vehicles (includes $558 measured at fair value)	572				0				572	0
Assets	883				521				883	521
Unearned premium reserve	(39)				(51)				(39)	(51)
Loss and loss adjustment expense reserve	(41)				(48)				(41)	(48)
Financial guaranty variable interest entities’ liabilities with recourse, at fair value	367				517				367	517
Financial guaranty variable interest entities’ liabilities without recourse, at fair value	102				102				102	102
Liabilities of consolidated investment vehicles (includes $481 measured at fair value)	482				0				482	0
Liabilities	871				520				871	520
Redeemable noncontrolling interests in consolidated investment vehicles	7				0				7	0
Retained earnings	34				34				34	34
Accumulated other comprehensive income, net of tax of $71 and $38	(35)				(33)				(35)	(33)
Total shareholders' equity attributable to AGL	(1)				1				(1)	1
Nonredeemable noncontrolling interests	6				0				6	0
Total shareholders’ equity	5				1				5	1
Liabilities and Equity	$ 883				$ 521				883	521
Income Statement Related Disclosures [Abstract]
Net earned premiums									(18)	(12)	(15)
Net investment income									(4)	(4)	(5)
Fair value gains (losses) on financial guaranty variable interest entities									42	14	30
Other income (loss)									(3)	0	0
Loss and LAE									20	3	(7)
Equity in net earnings of investees									2	0	0
Income (loss) before income taxes and equity in net earnings of investees									(1)	(5)	17
Provision (benefit) for income taxes									0	(1)	6
Net income (loss)									(1)	(4)	11
Less: noncontrolling interests									(1)	0	0
Net income (loss) attributable to Assured Guaranty Ltd.									0	(4)	11
Supplemental cash flow information
Effect on cash flows from operating activities									(254)	11	19
Effect on cash flows from investing activities									259	105	138
Effect on cash flows from financing activities									9	(116)	(157)
Increase (decrease) in cash and restricted cash									$ 14	$ 0	$ 0