Segment Information - Segment Information (Details) $ in Millions	3 Months Ended								9 Months Ended	12 Months Ended
	Dec. 31, 2019 USD ($) segment	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2019 segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment	2								1
Net investment income	$ 82	$ 88	$ 110	$ 98	$ 98	$ 99	$ 98	$ 100		$ 378	$ 395	$ 417
Interest expense	22	22	22	23	23	23	24	24		89	94	97
Non-cash compensation and operating expenses										48	41	41
Revenue, ancluding adjustments										964	959	1,558
Total revenues										964	959	1,558
Total expenses										516	431	705
Income (loss) before income taxes and equity in net earnings of investees										448	528	853
Equity in net earnings of investees	1	0	1	2	1	(1)	1	0		4	1	0
Adjusted operating income (loss) before income taxes										452	529	853
Provision (benefit) for income taxes										62	47	192
Less: noncontrolling interests	(1)	0	0	0	0	0	0	0		(1)	0	0
Adjusted Income (Loss) Attributable to Parent										391	482	661
Net realized investment gains (losses)	10	16	8	(12)	(18)	(7)	(2)	(5)		22	(32)	40
Unrealized Gain (Loss) on Derivatives and Commodity Contracts										(10)	101	43
Fair value gains (losses) on CSS										(22)	14	(2)
Less: Foreign exchange gains (losses) on remeasurement of premiums receivable and loss and LAE reserves										22	(32)	57
Pre-tax Adjustments to Operating Income										12	51	138
Pre-tax Adjustments to Operating Income, Tax										(1)	(12)	(69)
Net income (loss) attributable to Assured Guaranty Ltd.	$ 137	$ 69	$ 142	$ 54	$ 88	$ 161	$ 75	$ 197		402	521	730
United States [Member]
Segment Reporting Information [Line Items]
Total revenues										761	732	1,193
Bermuda [Member]
Segment Reporting Information [Line Items]
Total revenues										161	203	224
United Kingdom and Other [Member]
Segment Reporting Information [Line Items]
Total revenues										42	24	141
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Net investment income										4	6	2
Interest expense										94	97	100
Non-cash compensation and operating expenses										6	6	5
Revenue, ancluding adjustments										3	(28)	(8)
Total revenues										3	(28)	(8)
Total expenses										133	129	129
Income (loss) before income taxes and equity in net earnings of investees										(130)	(157)	(137)
Equity in net earnings of investees										0	0	0
Adjusted operating income (loss) before income taxes										(130)	(157)	(137)
Provision (benefit) for income taxes										(19)	(61)	(54)
Less: noncontrolling interests										0	0	0
Adjusted Income (Loss) Attributable to Parent										(111)	(96)	(83)
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Net investment income										(9)	(7)	(8)
Interest expense										(5)	(3)	(3)
Non-cash compensation and operating expenses										0	0	0
Revenue, ancluding adjustments										27	(2)	10
Total revenues										22	(5)	7
Total expenses										25	0	(10)
Income (loss) before income taxes and equity in net earnings of investees										(3)	(5)	17
Equity in net earnings of investees										2	0	0
Adjusted operating income (loss) before income taxes										(1)	(5)	17
Provision (benefit) for income taxes										0	(1)	5
Less: noncontrolling interests										(1)	0	0
Adjusted Income (Loss) Attributable to Parent										0	(4)	12
Net realized investment gains (losses)										22	(32)	40
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue, ancluding adjustments										(5)	(3)	(3)
Insurance Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net investment income										383	396	423
Interest expense										0	0	0
Non-cash compensation and operating expenses										39	35	36
Revenue, ancluding adjustments										912	989	1,556
Total revenues										917	992	1,559
Total expenses										324	302	586
Income (loss) before income taxes and equity in net earnings of investees										593	690	973
Equity in net earnings of investees										2	1	0
Adjusted operating income (loss) before income taxes										595	691	973
Provision (benefit) for income taxes										83	109	241
Less: noncontrolling interests										0	0	0
Adjusted Income (Loss) Attributable to Parent										512	582	732
Insurance Segment [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue, ancluding adjustments										5	3	3
Asset Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net investment income										0	0	0
Interest expense										0	0	0
Non-cash compensation and operating expenses										3	0	0
Revenue, ancluding adjustments										22	0	0
Total revenues										22	0	0
Total expenses										34	0	0
Income (loss) before income taxes and equity in net earnings of investees										(12)	0	0
Equity in net earnings of investees										0	0	0
Adjusted operating income (loss) before income taxes										(12)	0	0
Provision (benefit) for income taxes										(2)	0	0
Less: noncontrolling interests										0	0	0
Adjusted Income (Loss) Attributable to Parent										$ (10)	$ 0	$ 0