Subsidiary Information - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	$ 354	$ (189)
Fixed-maturity securities:
Purchases	(1,615)	(1,028)
Sales	1,128	877
Maturities	689	861
Net sales (purchases) of short-term investments	(240)	80
Net proceeds from financial guaranty variable entities’ assets	117	590
Investment in subsidiary	0	0
Acquisition of CIFG, net of cash acquired	0	(435)
Proceeds from sale of subsidiary	0
Payment to acquire business	95
Other	58	(12)
Net cash flows provided by (used in) investing activities	232	933
Cash flows from financing activities
Proceeds from Contributed Capital	0
Return of capital	0	0
Dividends paid	(53)	(52)
Repurchases of common stock	(431)	(190)
Repurchases of common stock to pay withholding taxes	(13)	(2)
Net paydowns of financial guaranty variable interest entities’ liabilities	(124)	(567)
Repayment/ extinguishment of long-term debt	(29)	(2)
Proceeds from option exercises	5	6
Net cash flows provided by (used in) financing activities	(645)	(807)
Effect of foreign exchange rate changes	4	(4)
Increase (decrease) in cash	(55)	(67)
Cash and Restricted Cash	72	99	$ 127	$ 166
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	503	288
Fixed-maturity securities:
Purchases	0	(4)
Sales	0	4
Maturities	0	0
Net sales (purchases) of short-term investments	(11)	(49)
Net proceeds from financial guaranty variable entities’ assets	0	0
Investment in subsidiary	0	0
Acquisition of CIFG, net of cash acquired		0
Proceeds from sale of subsidiary	0
Payment to acquire business	0
Other	0	0
Net cash flows provided by (used in) investing activities	(11)	(49)
Cash flows from financing activities
Proceeds from Contributed Capital	0
Return of capital	0	0
Dividends paid	(53)	(52)
Repurchases of common stock	(431)	(190)
Repurchases of common stock to pay withholding taxes	(13)	(2)
Net paydowns of financial guaranty variable interest entities’ liabilities	0	0
Repayment/ extinguishment of long-term debt	0	0
Proceeds from option exercises	5	6
Net cash flows provided by (used in) financing activities	(492)	(238)
Effect of foreign exchange rate changes	0	0
Increase (decrease) in cash	0	1
Cash and Restricted Cash	0	1	0	0
Reportable Legal Entities [Member] | Issuer Subsidiary [Member] | AGUS [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	175	171
Fixed-maturity securities:
Purchases	(75)	(44)
Sales	112	0
Maturities	7	23
Net sales (purchases) of short-term investments	218	(3)
Net proceeds from financial guaranty variable entities’ assets	0	0
Investment in subsidiary	(28)	0
Acquisition of CIFG, net of cash acquired		0
Proceeds from sale of subsidiary	0
Payment to acquire business	0
Other	0	7
Net cash flows provided by (used in) investing activities	234	(17)
Cash flows from financing activities
Proceeds from Contributed Capital	0
Return of capital	0	0
Dividends paid	(390)	(223)
Repurchases of common stock	0	0
Repurchases of common stock to pay withholding taxes	0	0
Net paydowns of financial guaranty variable interest entities’ liabilities	0	0
Repayment/ extinguishment of long-term debt	0	0
Proceeds from option exercises	0	0
Net cash flows provided by (used in) financing activities	(390)	(223)
Effect of foreign exchange rate changes	0	0
Increase (decrease) in cash	19	(69)
Cash and Restricted Cash	20	26	1	95
Reportable Legal Entities [Member] | Issuer Subsidiary [Member] | AGMH [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	105	159
Fixed-maturity securities:
Purchases	(15)	(10)
Sales	12	12
Maturities	0	0
Net sales (purchases) of short-term investments	3	(1)
Net proceeds from financial guaranty variable entities’ assets	0	0
Investment in subsidiary	0	0
Acquisition of CIFG, net of cash acquired		0
Proceeds from sale of subsidiary	0
Payment to acquire business	0
Other	0	0
Net cash flows provided by (used in) investing activities	0	1
Cash flows from financing activities
Proceeds from Contributed Capital	25
Return of capital	0	0
Dividends paid	(128)	(158)
Repurchases of common stock	0	0
Repurchases of common stock to pay withholding taxes	0	0
Net paydowns of financial guaranty variable interest entities’ liabilities	0	0
Repayment/ extinguishment of long-term debt	0	0
Proceeds from option exercises	0	0
Net cash flows provided by (used in) financing activities	(103)	(158)
Effect of foreign exchange rate changes	0	0
Increase (decrease) in cash	2	2
Cash and Restricted Cash	2	10	0	8
Reportable Legal Entities [Member] | Other Entities [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	469	3
Fixed-maturity securities:
Purchases	(1,552)	(970)
Sales	1,004	861
Maturities	682	838
Net sales (purchases) of short-term investments	(450)	133
Net proceeds from financial guaranty variable entities’ assets	117	590
Investment in subsidiary	(69)	4
Acquisition of CIFG, net of cash acquired		(442)
Proceeds from sale of subsidiary	139
Payment to acquire business	95
Other	58	(12)
Net cash flows provided by (used in) investing activities	24	1,002
Cash flows from financing activities
Proceeds from Contributed Capital	3
Return of capital	(70)	(4)
Dividends paid	(380)	(429)
Repurchases of common stock	0	0
Repurchases of common stock to pay withholding taxes	0	0
Net paydowns of financial guaranty variable interest entities’ liabilities	(124)	(567)
Repayment/ extinguishment of long-term debt	(2)	(2)
Proceeds from option exercises	0	0
Net cash flows provided by (used in) financing activities	(573)	(1,002)
Effect of foreign exchange rate changes	4	(4)
Increase (decrease) in cash	(76)	(1)
Cash and Restricted Cash	50	62	126	63
Consolidation, Eliminations [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	(898)	(810)
Fixed-maturity securities:
Purchases	27	0
Sales	0	0
Maturities	0	0
Net sales (purchases) of short-term investments	0	0
Net proceeds from financial guaranty variable entities’ assets	0	0
Investment in subsidiary	97	(4)
Acquisition of CIFG, net of cash acquired		7
Proceeds from sale of subsidiary	(139)
Payment to acquire business	0
Other	0	(7)
Net cash flows provided by (used in) investing activities	(15)	(4)
Cash flows from financing activities
Proceeds from Contributed Capital	(28)
Return of capital	70	4
Dividends paid	898	810
Repurchases of common stock	0	0
Repurchases of common stock to pay withholding taxes	0	0
Net paydowns of financial guaranty variable interest entities’ liabilities	0	0
Repayment/ extinguishment of long-term debt	(27)	0
Proceeds from option exercises	0	0
Net cash flows provided by (used in) financing activities	913	814
Effect of foreign exchange rate changes	0	0
Increase (decrease) in cash	0	0
Cash and Restricted Cash	$ 0	$ 0	$ 0	$ 0