Subsidiary Information - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	$ 462	$ 433	$ (132)
Fixed-maturity securities:
Purchases	(1,881)	(2,552)	(1,646)
Sales	1,180	1,701	1,365
Maturities	962	821	1,155
Short-term investments with maturities of over three months:
Purchases	(243)	(255)	(190)
Sales	23	102	172
Maturities	207	191	134
Net sales (purchases) of short-term investments with maturities of less than three months	(84)	36	(99)
Net proceeds from FG VIEs’ assets	116	147	629
Investment in subsidiaries	0	0
Intercompany debt	0	0	0
Proceeds from sale of subsidiaries		0
Return of capital from subsidiary	0	0	0
Acquisition, net of cash acquired	0	95	(435)
Other	17	59	(9)
Net cash flows provided by (used in) investing activities	297	345	1,076
Cash flows from financing activities
Return of capital		0	0
Capital contribution	0	0
Dividends paid	(71)	(70)	(69)
Repurchases of common stock	(500)	(501)	(306)
Repurchases of common stock to pay withholding taxes	(13)	(13)	(2)
Net paydowns of FG VIEs’ liabilities	(116)	(157)	(611)
Paydown of long-term debt	(101)	(30)	(2)
Proceeds from option exercises	6	5	12
Intercompany debt	0	0	0
Net cash flows provided by (used in) financing activities	(795)	(766)	(978)
Effect of foreign exchange rate changes	(4)	5	(5)
Increase (decrease) in cash and restricted cash	(40)	17	(39)
Cash and restricted cash at beginning of period (see Note 10)	144	127	166
Cash and restricted cash at end of period (see Note 10)	104	144	127
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	587	579	391
Fixed-maturity securities:
Purchases	0	0	(4)
Sales	0	0	4
Maturities	0	0	0
Short-term investments with maturities of over three months:
Purchases	0	0	0
Sales	0	0	0
Maturities	0	0	0
Net sales (purchases) of short-term investments with maturities of less than three months	(9)	0	(26)
Net proceeds from FG VIEs’ assets	0	0	0
Investment in subsidiaries	0	0
Intercompany debt	0	0	0
Proceeds from sale of subsidiaries		0
Return of capital from subsidiary	0	0	0
Acquisition, net of cash acquired		0	0
Other	0	0	0
Net cash flows provided by (used in) investing activities	(9)	0	(26)
Cash flows from financing activities
Return of capital		0	0
Capital contribution	0	0
Dividends paid	(71)	(70)	(69)
Repurchases of common stock	(500)	(501)	(306)
Repurchases of common stock to pay withholding taxes	(13)	(13)	(2)
Net paydowns of FG VIEs’ liabilities	0	0	0
Paydown of long-term debt	0	0	0
Proceeds from option exercises	6	5	12
Intercompany debt	0	0	0
Net cash flows provided by (used in) financing activities	(578)	(579)	(365)
Effect of foreign exchange rate changes	0	0	0
Increase (decrease) in cash and restricted cash	0	0	0
Cash and restricted cash at beginning of period (see Note 10)	0	0	0
Cash and restricted cash at end of period (see Note 10)	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	308	442	533
Fixed-maturity securities:
Purchases	(104)	(158)	(143)
Sales	104	112	24
Maturities	28	13	30
Short-term investments with maturities of over three months:
Purchases	(34)	(26)	(19)
Sales	22	1	0
Maturities	0	30	14
Net sales (purchases) of short-term investments with maturities of less than three months	(50)	126	(232)
Net proceeds from FG VIEs’ assets	0	0	0
Investment in subsidiaries	(9)	(28)
Intercompany debt	0	0	0
Proceeds from sale of subsidiaries		0
Return of capital from subsidiary	200	0	0
Acquisition, net of cash acquired		0	0
Other	(15)	0	7
Net cash flows provided by (used in) investing activities	142	70	(319)
Cash flows from financing activities
Return of capital		0	0
Capital contribution	0	0
Dividends paid	(472)	(470)	(288)
Repurchases of common stock	0	0	0
Repurchases of common stock to pay withholding taxes	0	0	0
Net paydowns of FG VIEs’ liabilities	0	0	0
Paydown of long-term debt	0	0	0
Proceeds from option exercises	0	0	0
Intercompany debt	(10)	(10)	(20)
Net cash flows provided by (used in) financing activities	(482)	(480)	(308)
Effect of foreign exchange rate changes	0	0	0
Increase (decrease) in cash and restricted cash	(32)	32	(94)
Cash and restricted cash at beginning of period (see Note 10)	33	1	95
Cash and restricted cash at end of period (see Note 10)	1	33	1
Reportable Legal Entities [Member] | AGMH (Issuer) [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	183	158	213
Fixed-maturity securities:
Purchases	(12)	(17)	(10)
Sales	8	21	12
Maturities	0	0	0
Short-term investments with maturities of over three months:
Purchases	0	(5)	(1)
Sales	0	5	0
Maturities	0	0	1
Net sales (purchases) of short-term investments with maturities of less than three months	7	(8)	(10)
Net proceeds from FG VIEs’ assets	0	0	0
Investment in subsidiaries	(1)	0
Intercompany debt	0	0	0
Proceeds from sale of subsidiaries		0
Return of capital from subsidiary	0	101	300
Acquisition, net of cash acquired		0	0
Other	0	0	0
Net cash flows provided by (used in) investing activities	2	97	292
Cash flows from financing activities
Return of capital		0	0
Capital contribution	0	25
Dividends paid	(187)	(278)	(513)
Repurchases of common stock	0	0	0
Repurchases of common stock to pay withholding taxes	0	0	0
Net paydowns of FG VIEs’ liabilities	0	0	0
Paydown of long-term debt	0	0	0
Proceeds from option exercises	0	0	0
Intercompany debt	0	0	0
Net cash flows provided by (used in) financing activities	(187)	(253)	(513)
Effect of foreign exchange rate changes	0	0	0
Increase (decrease) in cash and restricted cash	(2)	2	(8)
Cash and restricted cash at beginning of period (see Note 10)	2	0	8
Cash and restricted cash at end of period (see Note 10)	0	2	0
Reportable Legal Entities [Member] | Other Entities [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	517	477	72
Fixed-maturity securities:
Purchases	(1,865)	(2,404)	(1,489)
Sales	1,068	1,568	1,325
Maturities	934	808	1,125
Short-term investments with maturities of over three months:
Purchases	(209)	(224)	(170)
Sales	1	96	172
Maturities	207	161	119
Net sales (purchases) of short-term investments with maturities of less than three months	(32)	(82)	169
Net proceeds from FG VIEs’ assets	116	147	629
Investment in subsidiaries	(1)	(139)
Intercompany debt	10	10	20
Proceeds from sale of subsidiaries		139
Return of capital from subsidiary	0	70	4
Acquisition, net of cash acquired		95	(442)
Other	32	59	(9)
Net cash flows provided by (used in) investing activities	261	304	1,453
Cash flows from financing activities
Return of capital		(70)	(4)
Capital contribution	11	3
Dividends paid	(474)	(475)	(540)
Repurchases of common stock	(200)	(101)	(300)
Repurchases of common stock to pay withholding taxes	0	0	0
Net paydowns of FG VIEs’ liabilities	(116)	(157)	(611)
Paydown of long-term debt	(1)	(3)	(2)
Proceeds from option exercises	0	0	0
Intercompany debt	0	0	0
Net cash flows provided by (used in) financing activities	(780)	(803)	(1,457)
Effect of foreign exchange rate changes	(4)	5	(5)
Increase (decrease) in cash and restricted cash	(6)	(17)	63
Cash and restricted cash at beginning of period (see Note 10)	109	126	63
Cash and restricted cash at end of period (see Note 10)	103	109	126
Consolidating Adjustments [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	(1,133)	(1,223)	(1,341)
Fixed-maturity securities:
Purchases	100	27	0
Sales	0	0	0
Maturities	0	0	0
Short-term investments with maturities of over three months:
Purchases	0	0	0
Sales	0	0	0
Maturities	0	0	0
Net sales (purchases) of short-term investments with maturities of less than three months	0	0	0
Net proceeds from FG VIEs’ assets	0	0	0
Investment in subsidiaries	11	167
Intercompany debt	(10)	(10)	(20)
Proceeds from sale of subsidiaries		(139)
Return of capital from subsidiary	(200)	(171)	(304)
Acquisition, net of cash acquired		0	7
Other	0	0	(7)
Net cash flows provided by (used in) investing activities	(99)	(126)	(324)
Cash flows from financing activities
Return of capital		70	4
Capital contribution	(11)	(28)
Dividends paid	1,133	1,223	1,341
Repurchases of common stock	200	101	300
Repurchases of common stock to pay withholding taxes	0	0	0
Net paydowns of FG VIEs’ liabilities	0	0	0
Paydown of long-term debt	(100)	(27)	0
Proceeds from option exercises	0	0	0
Intercompany debt	10	10	20
Net cash flows provided by (used in) financing activities	1,232	1,349	1,665
Effect of foreign exchange rate changes	0	0	0
Increase (decrease) in cash and restricted cash	0	0	0
Cash and restricted cash at beginning of period (see Note 10)	0	0	0
Cash and restricted cash at end of period (see Note 10)	$ 0	$ 0	$ 0