Subsidiary Information - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	$ 433	$ (132)	$ (71)
Fixed-maturity securities:
Purchases	(2,552)	(1,646)	(2,577)
Sales	1,701	1,365	2,107
Maturities	821	1,155	898
Sales (purchases) of short-term investments, net	74	17	897
Net proceeds from FG VIE assets	147	629	400
Investment in subsidiaries	0
Intercompany debt	0	0
Proceeds from sale of subsidiaries	0
Proceeds from return of capital from subsidiaries	0	0	0
Acquisitions, net of cash acquired	95	(435)	(800)
Other	59	(9)	69
Net cash flows provided by (used in) investing activities	345	1,076	994
Cash flows from financing activities
Return of capital	0	0	0
Proceeds from Contributed Capital	0
Dividends paid	(70)	(69)	(72)
Repurchases of common stock	(501)	(306)	(555)
Repurchases of common stock to pay withholding taxes	(13)	(2)	(7)
Net paydowns of FG VIE liabilities	(157)	(611)	(214)
Paydown of long-term debt	(30)	(2)	(4)
Proceeds from option exercises	5	12	5
Intercompany debt	0	0
Net cash flows provided by (used in) financing activities	(766)	(978)	(847)
Effect of foreign exchange rate changes	5	(5)	(4)
Increase (Decrease) in Cash and Restricted Cash	17	(39)	72
Cash and Restricted Cash	144	127	166	$ 94
Reportable Legal Entities [Member] | Assured Guaranty Ltd. (Parent) [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	579	391	513
Fixed-maturity securities:
Purchases	0	(4)	0
Sales	0	4	0
Maturities	0	0	0
Sales (purchases) of short-term investments, net	0	(26)	116
Net proceeds from FG VIE assets	0	0	0
Investment in subsidiaries	0
Intercompany debt	0	0
Proceeds from sale of subsidiaries	0
Proceeds from return of capital from subsidiaries	0	0	0
Acquisitions, net of cash acquired	0	0	0
Other	0	0	0
Net cash flows provided by (used in) investing activities	0	(26)	116
Cash flows from financing activities
Return of capital	0	0	0
Proceeds from Contributed Capital	0
Dividends paid	(70)	(69)	(72)
Repurchases of common stock	(501)	(306)	(555)
Repurchases of common stock to pay withholding taxes	(13)	(2)	(7)
Net paydowns of FG VIE liabilities	0	0	0
Paydown of long-term debt	0	0	0
Proceeds from option exercises	5	12	5
Intercompany debt	0	0
Net cash flows provided by (used in) financing activities	(579)	(365)	(629)
Effect of foreign exchange rate changes	0	0	0
Increase (Decrease) in Cash and Restricted Cash	0	0	0
Cash and Restricted Cash	0	0	0	0
Reportable Legal Entities [Member] | AGUS (Issuer) [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	442	533	408
Fixed-maturity securities:
Purchases	(158)	(143)	(72)
Sales	112	24	177
Maturities	13	30	9
Sales (purchases) of short-term investments, net	131	(237)	33
Net proceeds from FG VIE assets	0	0	0
Investment in subsidiaries	(28)
Intercompany debt	0	0
Proceeds from sale of subsidiaries	0
Proceeds from return of capital from subsidiaries	0	0	0
Acquisitions, net of cash acquired	0	0	0
Other	0	7	(5)
Net cash flows provided by (used in) investing activities	70	(319)	142
Cash flows from financing activities
Return of capital	0	0	0
Proceeds from Contributed Capital	0
Dividends paid	(470)	(288)	(455)
Repurchases of common stock	0	0	0
Repurchases of common stock to pay withholding taxes	0	0	0
Net paydowns of FG VIE liabilities	0	0	0
Paydown of long-term debt	0	0	0
Proceeds from option exercises	0	0	0
Intercompany debt	(10)	(20)
Net cash flows provided by (used in) financing activities	(480)	(308)	(455)
Effect of foreign exchange rate changes	0	0	0
Increase (Decrease) in Cash and Restricted Cash	32	(94)	95
Cash and Restricted Cash	33	1	95	0
Reportable Legal Entities [Member] | AGMH (Issuer) [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	158	213	185
Fixed-maturity securities:
Purchases	(17)	(10)	(21)
Sales	21	12	30
Maturities	0	0	0
Sales (purchases) of short-term investments, net	(8)	(10)	19
Net proceeds from FG VIE assets	0	0	0
Investment in subsidiaries	0
Intercompany debt	0	0
Proceeds from sale of subsidiaries	0
Proceeds from return of capital from subsidiaries	101	300	25
Acquisitions, net of cash acquired	0	0	0
Other	0	0	0
Net cash flows provided by (used in) investing activities	97	292	53
Cash flows from financing activities
Return of capital	0	0	0
Proceeds from Contributed Capital	25
Dividends paid	(278)	(513)	(234)
Repurchases of common stock	0	0	0
Repurchases of common stock to pay withholding taxes	0	0	0
Net paydowns of FG VIE liabilities	0	0	0
Paydown of long-term debt	0	0	0
Proceeds from option exercises	0	0	0
Intercompany debt	0	0
Net cash flows provided by (used in) financing activities	(253)	(513)	(234)
Effect of foreign exchange rate changes	0	0	0
Increase (Decrease) in Cash and Restricted Cash	2	(8)	4
Cash and Restricted Cash	2	0	8	4
Reportable Legal Entities [Member] | Other Entities [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	477	72	33
Fixed-maturity securities:
Purchases	(2,404)	(1,489)	(2,550)
Sales	1,568	1,325	1,900
Maturities	808	1,125	889
Sales (purchases) of short-term investments, net	(49)	290	729
Net proceeds from FG VIE assets	147	629	400
Investment in subsidiaries	(139)
Intercompany debt	10	20
Proceeds from sale of subsidiaries	139
Proceeds from return of capital from subsidiaries	70	4	0
Acquisitions, net of cash acquired	95	(442)	(800)
Other	59	(9)	74
Net cash flows provided by (used in) investing activities	304	1,453	642
Cash flows from financing activities
Return of capital	(70)	(4)	(25)
Proceeds from Contributed Capital	3
Dividends paid	(475)	(540)	(455)
Repurchases of common stock	(101)	(300)	0
Repurchases of common stock to pay withholding taxes	0	0	0
Net paydowns of FG VIE liabilities	(157)	(611)	(214)
Paydown of long-term debt	(3)	(2)	(4)
Proceeds from option exercises	0	0	0
Intercompany debt	0	0
Net cash flows provided by (used in) financing activities	(803)	(1,457)	(698)
Effect of foreign exchange rate changes	5	(5)	(4)
Increase (Decrease) in Cash and Restricted Cash	(17)	63	(27)
Cash and Restricted Cash	109	126	63	90
Consolidation, Eliminations [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	(1,223)	(1,341)	(1,210)
Fixed-maturity securities:
Purchases	27	0	66
Sales	0	0	0
Maturities	0	0	0
Sales (purchases) of short-term investments, net	0	0	0
Net proceeds from FG VIE assets	0	0	0
Investment in subsidiaries	167
Intercompany debt	(10)	(20)
Proceeds from sale of subsidiaries	(139)
Proceeds from return of capital from subsidiaries	(171)	(304)	(25)
Acquisitions, net of cash acquired	0	7	0
Other	0	(7)	0
Net cash flows provided by (used in) investing activities	(126)	(324)	41
Cash flows from financing activities
Return of capital	70	4	25
Proceeds from Contributed Capital	(28)
Dividends paid	1,223	1,341	1,144
Repurchases of common stock	101	300	0
Repurchases of common stock to pay withholding taxes	0	0	0
Net paydowns of FG VIE liabilities	0	0	0
Paydown of long-term debt	(27)	0	0
Proceeds from option exercises	0	0	0
Intercompany debt	10	20
Net cash flows provided by (used in) financing activities	1,349	1,665	1,169
Effect of foreign exchange rate changes	0	0	0
Increase (Decrease) in Cash and Restricted Cash	0	0	0
Cash and Restricted Cash	$ 0	$ 0	$ 0	$ 0