Subsidiary Information - Condensed Consolidating Statement of Operations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Revenues
Net earned premiums	$ 118	$ 145
Net investment income	98	100
Net realized investment gains (losses)	(12)	(5)
Net change in fair value of credit derivatives	(22)	34
Other	13	19
Total revenues	195	293
Expenses
Loss and LAE	46	(18)
Amortization of deferred acquisition costs	6	5
Interest expense	23	24
Other operating expenses	64	65
Total expenses	139	76
Equity in net earnings of investees	2	0
Total before tax	58	217
Total (provision) benefit for income taxes	(4)	(20)
Equity in net earnings of subsidiaries	0	0
Net income (loss)	54	197
Less: noncontrolling interest	0	0
Net income (loss) attributable to Assured Guaranty Ltd.	54	197
Comprehensive income (loss)	222	71
Reportable Legal Entities [Member] | Assured Guaranty Ltd. (Parent) [Member]
Revenues
Net earned premiums	0	0
Net investment income	0	0
Net realized investment gains (losses)	0	0
Net change in fair value of credit derivatives	0	0
Other	2	3
Total revenues	2	3
Expenses
Loss and LAE	0	0
Amortization of deferred acquisition costs	0	0
Interest expense	0	0
Other operating expenses	10	10
Total expenses	10	10
Equity in net earnings of investees	0	0
Total before tax	(8)	(7)
Total (provision) benefit for income taxes	0	0
Equity in net earnings of subsidiaries	62	204
Net income (loss)	54	197
Less: noncontrolling interest	0	0
Net income (loss) attributable to Assured Guaranty Ltd.	54	197
Comprehensive income (loss)	222	71
Reportable Legal Entities [Member] | Issuer Subsidiary [Member] | AGUS [Member]
Revenues
Net earned premiums	0	0
Net investment income	2	2
Net realized investment gains (losses)	0	0
Net change in fair value of credit derivatives	0	0
Other	0	0
Total revenues	2	2
Expenses
Loss and LAE	0	0
Amortization of deferred acquisition costs	0	0
Interest expense	12	12
Other operating expenses	1	3
Total expenses	13	15
Equity in net earnings of investees	1	0
Total before tax	(10)	(13)
Total (provision) benefit for income taxes	2	3
Equity in net earnings of subsidiaries	50	161
Net income (loss)	42	151
Less: noncontrolling interest	0	0
Net income (loss) attributable to Assured Guaranty Ltd.	42	151
Comprehensive income (loss)	170	67
Reportable Legal Entities [Member] | Issuer Subsidiary [Member] | AGMH [Member]
Revenues
Net earned premiums	0	0
Net investment income	0	0
Net realized investment gains (losses)	0	0
Net change in fair value of credit derivatives	0	0
Other	0	0
Total revenues	0	0
Expenses
Loss and LAE	0	0
Amortization of deferred acquisition costs	0	0
Interest expense	13	13
Other operating expenses	0	0
Total expenses	13	13
Equity in net earnings of investees	0	0
Total before tax	(13)	(13)
Total (provision) benefit for income taxes	3	3
Equity in net earnings of subsidiaries	68	102
Net income (loss)	58	92
Less: noncontrolling interest	0	0
Net income (loss) attributable to Assured Guaranty Ltd.	58	92
Comprehensive income (loss)	152	46
Reportable Legal Entities [Member] | Other Entities [Member]
Revenues
Net earned premiums	121	149
Net investment income	101	101
Net realized investment gains (losses)	(12)	(5)
Net change in fair value of credit derivatives	(22)	34
Other	66	74
Total revenues	254	353
Expenses
Loss and LAE	47	(16)
Amortization of deferred acquisition costs	7	6
Interest expense	4	3
Other operating expenses	107	105
Total expenses	165	98
Equity in net earnings of investees	1	0
Total before tax	90	255
Total (provision) benefit for income taxes	(9)	(25)
Equity in net earnings of subsidiaries	4	7
Net income (loss)	85	237
Less: noncontrolling interest	4	7
Net income (loss) attributable to Assured Guaranty Ltd.	81	230
Comprehensive income (loss)	254	111
Consolidating Adjustments [Member]
Revenues
Net earned premiums	(3)	(4)
Net investment income	(5)	(3)
Net realized investment gains (losses)	0	0
Net change in fair value of credit derivatives	0	0
Other	(55)	(58)
Total revenues	(63)	(65)
Expenses
Loss and LAE	(1)	(2)
Amortization of deferred acquisition costs	(1)	(1)
Interest expense	(6)	(4)
Other operating expenses	(54)	(53)
Total expenses	(62)	(60)
Equity in net earnings of investees	0	0
Total before tax	(1)	(5)
Total (provision) benefit for income taxes	0	(1)
Equity in net earnings of subsidiaries	(184)	(474)
Net income (loss)	(185)	(480)
Less: noncontrolling interest	(4)	(7)
Net income (loss) attributable to Assured Guaranty Ltd.	(181)	(473)
Comprehensive income (loss)	$ (576)	$ (224)