Subsidiary Information - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	$ 444	$ 198
Fixed-maturity securities:
Purchases	(879)	(1,143)
Sales	592	778
Maturities	533	462
Net sales (purchases) of short-term investments	(304)	20
Net proceeds from financial guaranty variable entities’ assets	60	81
Investment in subsidiary		0
Proceeds from stock redemption and return of capital from subsidiaries	0
Proceeds from sale of subsidiaries		0
Acquisition of MBIA UK, net of cash acquired		95
Other	(16)	68
Net cash flows provided by (used in) investing activities	(14)	361
Cash flows from financing activities
Return of capital	0
Capital contribution	0	0
Dividends paid	(37)	(36)
Repurchases of common stock	(250)	(351)
Repurchases of common stock to pay withholding taxes	(13)	(12)
Net paydowns of financial guaranty variable interest entities’ liabilities	(61)	(86)
Paydown of long-term debt	(24)	(6)
Proceeds from option exercises	1	3
Net cash flows provided by (used in) financing activities	(384)	(488)
Effect of foreign exchange rate changes	(1)	3
Increase (decrease) in cash	45	74
Cash and restricted cash at beginning of period	144	127	$ 127
Cash and restricted cash at end of period	189	201	144
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	296	387
Fixed-maturity securities:
Purchases	0	0
Sales	0	0
Maturities	0	0
Net sales (purchases) of short-term investments	3	9
Net proceeds from financial guaranty variable entities’ assets	0	0
Investment in subsidiary		0
Proceeds from stock redemption and return of capital from subsidiaries	0
Proceeds from sale of subsidiaries		0
Acquisition of MBIA UK, net of cash acquired		0
Other	0	0
Net cash flows provided by (used in) investing activities	3	9
Cash flows from financing activities
Return of capital	0
Capital contribution	0	0
Dividends paid	(37)	(36)
Repurchases of common stock	(250)	(351)
Repurchases of common stock to pay withholding taxes	(13)	(12)
Net paydowns of financial guaranty variable interest entities’ liabilities	0	0
Paydown of long-term debt	0	0
Proceeds from option exercises	1	3
Net cash flows provided by (used in) financing activities	(299)	(396)
Effect of foreign exchange rate changes	0	0
Increase (decrease) in cash	0	0
Cash and restricted cash at beginning of period	0	0	0
Cash and restricted cash at end of period	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	88	109
Fixed-maturity securities:
Purchases	(27)	(54)
Sales	11	79
Maturities	10	7
Net sales (purchases) of short-term investments	(78)	206
Net proceeds from financial guaranty variable entities’ assets	0	0
Investment in subsidiary		(28)
Proceeds from stock redemption and return of capital from subsidiaries	200
Proceeds from sale of subsidiaries		0
Acquisition of MBIA UK, net of cash acquired		0
Other	(15)	0
Net cash flows provided by (used in) investing activities	101	210
Cash flows from financing activities
Return of capital	0
Capital contribution	0	0
Dividends paid	(222)	(320)
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
Paydown of long-term debt	0	0
Proceeds from option exercises	0	0
Net cash flows provided by (used in) financing activities	(222)	(320)
Effect of foreign exchange rate changes	0	0
Increase (decrease) in cash	(33)	(1)
Cash and restricted cash at beginning of period	33	1	1
Cash and restricted cash at end of period	0	0	33
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	43	55
Fixed-maturity securities:
Purchases	(12)	(5)
Sales	2	6
Maturities	0	0
Net sales (purchases) of short-term investments	16	(8)
Net proceeds from financial guaranty variable entities’ assets	0	0
Investment in subsidiary		0
Proceeds from stock redemption and return of capital from subsidiaries	0
Proceeds from sale of subsidiaries		0
Acquisition of MBIA UK, net of cash acquired		0
Other	0	0
Net cash flows provided by (used in) investing activities	6	(7)
Cash flows from financing activities
Return of capital	0
Capital contribution	0	25
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
Paydown of long-term debt	0	0
Proceeds from option exercises	0	0
Net cash flows provided by (used in) financing activities	(50)	(48)
Effect of foreign exchange rate changes	0	0
Increase (decrease) in cash	(1)	0
Cash and restricted cash at beginning of period	2	0	0
Cash and restricted cash at end of period	1	0	2
Reportable Legal Entities [Member] | Other Entities [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	524	264
Fixed-maturity securities:
Purchases	(863)	(1,089)
Sales	579	693
Maturities	523	455
Net sales (purchases) of short-term investments	(245)	(187)
Net proceeds from financial guaranty variable entities’ assets	60	81
Investment in subsidiary		(139)
Proceeds from stock redemption and return of capital from subsidiaries	0
Proceeds from sale of subsidiaries		139
Acquisition of MBIA UK, net of cash acquired		95
Other	(1)	68
Net cash flows provided by (used in) investing activities	53	116
Cash flows from financing activities
Return of capital	0
Capital contribution	0	3
Dividends paid	(235)	(224)
Repurchases of common stock	(200)	0
Repurchases of common stock to pay withholding taxes	0	0
Net paydowns of financial guaranty variable interest entities’ liabilities	(61)	(86)
Paydown of long-term debt	(1)	(1)
Proceeds from option exercises	0	0
Net cash flows provided by (used in) financing activities	(497)	(308)
Effect of foreign exchange rate changes	(1)	3
Increase (decrease) in cash	79	75
Cash and restricted cash at beginning of period	109	126	126
Cash and restricted cash at end of period	188	201	109
Consolidation, Eliminations [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	(507)	(617)
Fixed-maturity securities:
Purchases	23	5
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		167
Proceeds from stock redemption and return of capital from subsidiaries	(200)
Proceeds from sale of subsidiaries		(139)
Acquisition of MBIA UK, net of cash acquired		0
Other	0	0
Net cash flows provided by (used in) investing activities	(177)	33
Cash flows from financing activities
Return of capital	0
Capital contribution	0	(28)
Dividends paid	507	617
Repurchases of common stock	200	0
Repurchases of common stock to pay withholding taxes	0	0
Net paydowns of financial guaranty variable interest entities’ liabilities	0	0
Paydown of long-term debt	(23)	(5)
Proceeds from option exercises	0	0
Net cash flows provided by (used in) financing activities	684	584
Effect of foreign exchange rate changes	0	0
Increase (decrease) in cash	0	0
Cash and restricted cash at beginning of period	0	0	0
Cash and restricted cash at end of period	$ 0	$ 0	$ 0