Subsidiary Information - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	$ 27	$ 102
Fixed-maturity securities:
Purchases	(411)	(517)
Sales	409	323
Maturities	225	265
Net sales (purchases) of short-term investments	(116)	12
Net proceeds from financial guaranty variable entities’ assets	33	46
Investment in subsidiary	0	0
Proceeds from stock redemption and return of capital from subsidiaries	0
Acquisition of CIFG, net of cash acquired		95
Other	(14)	(13)
Net cash flows provided by (used in) investing activities	126	211
Cash flows from financing activities
Proceeds from Contributed Capital	0
Return of capital	0	0
Dividends paid	(18)	(19)
Repurchases of common stock	(100)	(216)
Repurchases of common stock to pay withholding taxes	(12)	(12)
Net paydowns of financial guaranty variable interest entities’ liabilities	(33)	(48)
Repayment/ extinguishment of long-term debt	(19)	(1)
Proceeds from option exercises	1	2
Net cash flows provided by (used in) financing activities	(181)	(294)
Effect of foreign exchange rate changes	1	2
Increase (decrease) in cash	(27)	21
Cash and Restricted Cash	117	148	$ 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	128	220
Fixed-maturity securities:
Purchases	0	0
Sales	0	0
Maturities	0	0
Net sales (purchases) of short-term investments	1	25
Net proceeds from financial guaranty variable entities’ assets	0	0
Investment in subsidiary	0	0
Proceeds from stock redemption and return of capital from subsidiaries	0
Acquisition of CIFG, net of cash acquired		0
Other	0	0
Net cash flows provided by (used in) investing activities	1	25
Cash flows from financing activities
Proceeds from Contributed Capital	0
Return of capital	0	0
Dividends paid	(18)	(19)
Repurchases of common stock	(100)	(216)
Repurchases of common stock to pay withholding taxes	(12)	(12)
Net paydowns of financial guaranty variable interest entities’ liabilities	0	0
Repayment/ extinguishment of long-term debt	0	0
Proceeds from option exercises	1	2
Net cash flows provided by (used in) financing activities	(129)	(245)
Effect of foreign exchange rate changes	0	0
Increase (decrease) in cash	0	0
Cash and Restricted Cash	0	0	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	83	105
Fixed-maturity securities:
Purchases	(18)	0
Sales	11	0
Maturities	0	3
Net sales (purchases) of short-term investments	(217)	75
Net proceeds from financial guaranty variable entities’ assets	0	0
Investment in subsidiary	0	(3)
Proceeds from stock redemption and return of capital from subsidiaries	200
Acquisition of CIFG, net of cash acquired		0
Other	(13)	0
Net cash flows provided by (used in) investing activities	(37)	75
Cash flows from financing activities
Proceeds from Contributed Capital	0
Return of capital	0	0
Dividends paid	(78)	(175)
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	(78)	(175)
Effect of foreign exchange rate changes	0	0
Increase (decrease) in cash	(32)	5
Cash and Restricted Cash	1	6	33	1
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	63	72
Fixed-maturity securities:
Purchases	(12)	(5)
Sales	2	2
Maturities	0	0
Net sales (purchases) of short-term investments	(4)	5
Net proceeds from financial guaranty variable entities’ assets	0	0
Investment in subsidiary	0	0
Proceeds from stock redemption and return of capital from subsidiaries	0
Acquisition of CIFG, net of cash acquired		0
Other	0	0
Net cash flows provided by (used in) investing activities	(14)	2
Cash flows from financing activities
Proceeds from Contributed Capital	0
Return of capital	0	0
Dividends paid	(50)	(73)
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	(50)	(73)
Effect of foreign exchange rate changes	0	0
Increase (decrease) in cash	(1)	1
Cash and Restricted Cash	1	1	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	46	105
Fixed-maturity securities:
Purchases	(400)	(512)
Sales	396	321
Maturities	225	262
Net sales (purchases) of short-term investments	104	(93)
Net proceeds from financial guaranty variable entities’ assets	33	46
Investment in subsidiary	0	0
Proceeds from stock redemption and return of capital from subsidiaries	0
Acquisition of CIFG, net of cash acquired		95
Other	(1)	(13)
Net cash flows provided by (used in) investing activities	357	106
Cash flows from financing activities
Proceeds from Contributed Capital	0
Return of capital	0	3
Dividends paid	(165)	(152)
Repurchases of common stock	0	0
Repurchases of common stock to pay withholding taxes	(200)	0
Net paydowns of financial guaranty variable interest entities’ liabilities	(33)	(48)
Repayment/ extinguishment of long-term debt	0	(1)
Proceeds from option exercises	0	0
Net cash flows provided by (used in) financing activities	(398)	(198)
Effect of foreign exchange rate changes	1	2
Increase (decrease) in cash	6	15
Cash and Restricted Cash	115	141	109	126
Consolidation, Eliminations [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	(293)	(400)
Fixed-maturity securities:
Purchases	19	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	0	3
Proceeds from stock redemption and return of capital from subsidiaries	(200)
Acquisition of CIFG, net of cash acquired		0
Other	0	0
Net cash flows provided by (used in) investing activities	(181)	3
Cash flows from financing activities
Proceeds from Contributed Capital	0
Return of capital	0	(3)
Dividends paid	293	400
Repurchases of common stock	0	0
Repurchases of common stock to pay withholding taxes	200	0
Net paydowns of financial guaranty variable interest entities’ liabilities	0	0
Repayment/ extinguishment of long-term debt	(19)	0
Proceeds from option exercises	0	0
Net cash flows provided by (used in) financing activities	474	397
Effect of foreign exchange rate changes	0	0
Increase (decrease) in cash	0	0
Cash and Restricted Cash	$ 0	$ 0	$ 0	$ 0