Subsidiary Information - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities				$ 352	$ 354
Fixed-maturity securities:
Purchases				(1,478)	(1,615)
Sales	$ 316		$ 350	908	1,128
Maturities				746	689
Net sales (purchases) of short-term investments				(91)	(240)
Net proceeds from FG VIEs’ assets				90	117
Proceeds from sale of subsidiaries					0
Return of capital from subsidiary				0
Proceeds from sale of subsidiaries					0
Acquisition of MBIA UK, net of cash acquired					95
Other					(58)
Other				19	(27)
Net cash flows provided by (used in) investing activities				194	232
Cash flows from financing activities
Return of capital				0	0
Capital contribution				0	0
Dividends paid				(55)	(53)
Repurchases of common stock				(380)	(431)
Repurchases of common stock to pay withholding taxes				(13)	(13)
Net paydowns of financial guaranty variable interest entities’ liabilities				(90)	(124)
Paydown of long-term debt				(73)	(29)
Proceeds from option exercises				5	5
Net cash flows provided by (used in) financing activities				(606)	(645)
Effect of foreign exchange rate changes				(2)	4
Increase (decrease) in cash				(62)	(55)
Cash and restricted cash at beginning of period		$ 72		144	127	$ 127
Cash and restricted cash at end of period	82	144	72	82	72	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				454	503
Fixed-maturity securities:
Purchases				0	0
Sales				0	0
Maturities				0	0
Net sales (purchases) of short-term investments				(11)	(11)
Net proceeds from FG VIEs’ assets				0	0
Proceeds from sale of subsidiaries					0
Return of capital from subsidiary				0
Proceeds from sale of subsidiaries					0
Acquisition of MBIA UK, net of cash acquired					0
Other					0
Other				0
Net cash flows provided by (used in) investing activities				(11)	(11)
Cash flows from financing activities
Return of capital				0	0
Capital contribution				0	0
Dividends paid				(55)	(53)
Repurchases of common stock				(380)	(431)
Repurchases of common stock to pay withholding taxes				(13)	(13)
Net paydowns of financial guaranty variable interest entities’ liabilities				0	0
Paydown of long-term debt				0	0
Proceeds from option exercises				5	5
Net cash flows provided by (used in) financing activities				(443)	(492)
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	0
Cash and restricted cash at end of period	0	0	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				101	175
Fixed-maturity securities:
Purchases				(76)	(75)
Sales				31	112
Maturities				28	7
Net sales (purchases) of short-term investments				60	218
Net proceeds from FG VIEs’ assets				0	0
Proceeds from sale of subsidiaries					(28)
Return of capital from subsidiary				200
Proceeds from sale of subsidiaries					0
Acquisition of MBIA UK, net of cash acquired					0
Other					0
Other				(15)
Net cash flows provided by (used in) investing activities				228	234
Cash flows from financing activities
Return of capital				0	0
Capital contribution				0	0
Dividends paid				(362)	(390)
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				(362)	(390)
Effect of foreign exchange rate changes				0	0
Increase (decrease) in cash				(33)	19
Cash and restricted cash at beginning of period		20		33	1	1
Cash and restricted cash at end of period	0	33	20	0	20	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				126	105
Fixed-maturity securities:
Purchases				(12)	(15)
Sales				8	12
Maturities				0	0
Net sales (purchases) of short-term investments				(74)	3
Net proceeds from FG VIEs’ assets				0	0
Proceeds from sale of subsidiaries					0
Return of capital from subsidiary				0
Proceeds from sale of subsidiaries					0
Acquisition of MBIA UK, net of cash acquired					0
Other					0
Other				0
Net cash flows provided by (used in) investing activities				(78)	0
Cash flows from financing activities
Return of capital				0	0
Capital contribution				0	25
Dividends paid				(50)	(128)
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)	(103)
Effect of foreign exchange rate changes				0	0
Increase (decrease) in cash				(2)	2
Cash and restricted cash at beginning of period		2		2	0	0
Cash and restricted cash at end of period	0	2	2	0	2	2
Reportable Legal Entities [Member] | Other Entities [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities				448	469
Fixed-maturity securities:
Purchases				(1,462)	(1,552)
Sales				869	1,004
Maturities				718	682
Net sales (purchases) of short-term investments				(66)	(450)
Net proceeds from FG VIEs’ assets				90	117
Proceeds from sale of subsidiaries					(69)
Return of capital from subsidiary				0
Proceeds from sale of subsidiaries					139
Acquisition of MBIA UK, net of cash acquired					95
Other					(58)
Other				34
Net cash flows provided by (used in) investing activities				183	24
Cash flows from financing activities
Return of capital				0	(70)
Capital contribution				0	3
Dividends paid				(365)	(380)
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				(90)	(124)
Paydown of long-term debt				(1)	(2)
Proceeds from option exercises				0	0
Net cash flows provided by (used in) financing activities				(656)	(573)
Effect of foreign exchange rate changes				(2)	4
Increase (decrease) in cash				(27)	(76)
Cash and restricted cash at beginning of period		50		109	126	126
Cash and restricted cash at end of period	82	109	50	82	50	109
Consolidating Adjustments [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities				(777)	(898)
Fixed-maturity securities:
Purchases				72	27
Sales				0	0
Maturities				0	0
Net sales (purchases) of short-term investments				0	0
Net proceeds from FG VIEs’ assets				0	0
Proceeds from sale of subsidiaries					97
Return of capital from subsidiary				(200)
Proceeds from sale of subsidiaries					(139)
Acquisition of MBIA UK, net of cash acquired					0
Other					0
Other				0
Net cash flows provided by (used in) investing activities				(128)	(15)
Cash flows from financing activities
Return of capital				0	70
Capital contribution				0	(28)
Dividends paid				777	898
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				(72)	(27)
Proceeds from option exercises				0	0
Net cash flows provided by (used in) financing activities				905	913
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	0
Cash and restricted cash at end of period	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0