Subsidiary Information - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	$ (509)	$ 462	$ 433
Fixed-maturity securities:
Purchases	(873)	(1,881)	(2,552)
Sales	1,805	1,180	1,701
Maturities and paydowns	781	962	821
Short-term investments with maturities of over three months:
Purchases	(229)	(243)	(255)
Sales	2	23	102
Maturities and paydowns	316	207	191
Net sales (purchases) of short-term investments with maturities of less than three months	(623)	(84)	36
Net proceeds from paydowns on FG VIEs’ assets	139	116	147
Net proceeds from sales of financial guaranty variable interest entities’ assets	51	0	0
Investment in subsidiaries	0	0	0
Intercompany debt	0	0	0
Investing Activities, Issuance of Intercompany Loans	0
Proceeds from sale of subsidiaries			0
Return of capital from subsidiaries	0	0	0
Acquisition, net of cash acquired	(145)	0	95
Other	55	17	(59)
Net cash flows provided by (used in) investing activities	1,169	297	345
Cash flows from financing activities
Return of capital	0		0
Capital contribution	0	0	0
Dividends paid	(74)	(71)	(70)
Repurchases of common stock	(500)	(500)	(501)
Net paydowns of FG VIEs’ liabilities	(181)	(116)	(157)
Paydown of long-term debt	(4)	(101)	(30)
Intercompany debt	0	0	0
Issuance of intercompany loans	0
Proceeds from issuance of collateralized loan obligations	482
Repayment of warehouse loans and equity	(306)	0	0
Contributions from noncontrolling interests to investment vehicles	18	0	0
Distributions to redeemable noncontrolling interests from investment vehicles	(4)	0	0
Other	(15)	(7)	(8)
Net cash flows provided by (used in) financing activities	(584)	(795)	(766)
Effect of foreign exchange rate changes	3	(4)	5
Increase (decrease) in cash and restricted cash	79	(40)	17
Cash and restricted cash at beginning of period	104	144	127
Cash and restricted cash at end of period	183	104	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	679	587	579
Fixed-maturity securities:
Purchases	0	0	0
Sales	0	0	0
Maturities and paydowns	0	0	0
Short-term investments with maturities of over three months:
Purchases	0	0	0
Sales	0	0	0
Maturities and paydowns	0	0	0
Net sales (purchases) of short-term investments with maturities of less than three months	(90)	(9)	0
Net proceeds from paydowns on FG VIEs’ assets	0	0	0
Net proceeds from sales of financial guaranty variable interest entities’ assets	0
Investment in subsidiaries	0	0	0
Intercompany debt	0	0	0
Investing Activities, Issuance of Intercompany Loans	0
Proceeds from sale of subsidiaries			0
Return of capital from subsidiaries	0	0	0
Acquisition, net of cash acquired	0		0
Other	0	0	0
Net cash flows provided by (used in) investing activities	(90)	(9)	0
Cash flows from financing activities
Return of capital	0		0
Capital contribution	0	0	0
Dividends paid	(74)	(71)	(70)
Repurchases of common stock	(500)	(500)	(501)
Net paydowns of FG VIEs’ liabilities	0	0	0
Paydown of long-term debt	0	0	0
Intercompany debt	0	0	0
Issuance of intercompany loans	0
Proceeds from issuance of collateralized loan obligations	0
Repayment of warehouse loans and equity	0
Contributions from noncontrolling interests to investment vehicles	0
Distributions to redeemable noncontrolling interests from investment vehicles	0
Other	(15)	(7)	(8)
Net cash flows provided by (used in) financing activities	(589)	(578)	(579)
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	0	0	0
Cash and restricted cash at end of period	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	190	308	442
Fixed-maturity securities:
Purchases	(3)	(104)	(158)
Sales	0	104	112
Maturities and paydowns	11	28	13
Short-term investments with maturities of over three months:
Purchases	0	(34)	(26)
Sales	0	22	1
Maturities and paydowns	12	0	30
Net sales (purchases) of short-term investments with maturities of less than three months	(44)	(50)	126
Net proceeds from paydowns on FG VIEs’ assets	0	0	0
Net proceeds from sales of financial guaranty variable interest entities’ assets	0
Investment in subsidiaries	65	(9)	(28)
Intercompany debt	0	0	0
Investing Activities, Issuance of Intercompany Loans	0
Proceeds from sale of subsidiaries			0
Return of capital from subsidiaries	100	200	0
Acquisition, net of cash acquired	(157)		0
Other	0	(15)	0
Net cash flows provided by (used in) investing activities	(16)	142	70
Cash flows from financing activities
Return of capital	0		0
Capital contribution	0	0	0
Dividends paid	(414)	(472)	(470)
Repurchases of common stock	0	0	0
Net paydowns of FG VIEs’ liabilities	0	0	0
Paydown of long-term debt	0	0	0
Intercompany debt	(10)	(10)	(10)
Issuance of intercompany loans	250
Proceeds from issuance of collateralized loan obligations	0
Repayment of warehouse loans and equity	0
Contributions from noncontrolling interests to investment vehicles	0
Distributions to redeemable noncontrolling interests from investment vehicles	0
Other	0	0	0
Net cash flows provided by (used in) financing activities	(174)	(482)	(480)
Effect of foreign exchange rate changes	0	0	0
Increase (decrease) in cash and restricted cash	0	(32)	32
Cash and restricted cash at beginning of period	1	33	1
Cash and restricted cash at end of period	1	1	33
Reportable Legal Entities [Member] | AGMH (Issuer) [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	172	183	158
Fixed-maturity securities:
Purchases	0	(12)	(17)
Sales	0	8	21
Maturities and paydowns	8	0	0
Short-term investments with maturities of over three months:
Purchases	0	0	(5)
Sales	0	0	5
Maturities and paydowns	0	0	0
Net sales (purchases) of short-term investments with maturities of less than three months	4	7	(8)
Net proceeds from paydowns on FG VIEs’ assets	0	0	0
Net proceeds from sales of financial guaranty variable interest entities’ assets	0
Investment in subsidiaries	5	(1)	0
Intercompany debt	0	0	0
Investing Activities, Issuance of Intercompany Loans	0
Proceeds from sale of subsidiaries			0
Return of capital from subsidiaries	0	0	101
Acquisition, net of cash acquired	0		0
Other	0	0	0
Net cash flows provided by (used in) investing activities	17	2	97
Cash flows from financing activities
Return of capital	0		0
Capital contribution	0	0	25
Dividends paid	(186)	(187)	(278)
Repurchases of common stock	0	0	0
Net paydowns of FG VIEs’ liabilities	0	0	0
Paydown of long-term debt	0	0	0
Intercompany debt	0	0	0
Issuance of intercompany loans	0
Proceeds from issuance of collateralized loan obligations	0
Repayment of warehouse loans and equity	0
Contributions from noncontrolling interests to investment vehicles	0
Distributions to redeemable noncontrolling interests from investment vehicles	0
Other	0	0	0
Net cash flows provided by (used in) financing activities	(186)	(187)	(253)
Effect of foreign exchange rate changes	0	0	0
Increase (decrease) in cash and restricted cash	3	(2)	2
Cash and restricted cash at beginning of period	0	2	0
Cash and restricted cash at end of period	3	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	(287)	517	477
Fixed-maturity securities:
Purchases	(873)	(1,865)	(2,404)
Sales	1,805	1,068	1,568
Maturities and paydowns	762	934	808
Short-term investments with maturities of over three months:
Purchases	(229)	(209)	(224)
Sales	2	1	96
Maturities and paydowns	304	207	161
Net sales (purchases) of short-term investments with maturities of less than three months	(493)	(32)	(82)
Net proceeds from paydowns on FG VIEs’ assets	139	116	147
Net proceeds from sales of financial guaranty variable interest entities’ assets	51
Investment in subsidiaries	(175)	(1)	(139)
Intercompany debt	10	10	10
Investing Activities, Issuance of Intercompany Loans	(250)
Proceeds from sale of subsidiaries			139
Return of capital from subsidiaries	10	0	70
Acquisition, net of cash acquired	12		95
Other	55	32	(59)
Net cash flows provided by (used in) investing activities	1,020	261	304
Cash flows from financing activities
Return of capital	(10)		(70)
Capital contribution	105	11	3
Dividends paid	(649)	(474)	(475)
Repurchases of common stock	(100)	(200)	(101)
Net paydowns of FG VIEs’ liabilities	(181)	(116)	(157)
Paydown of long-term debt	(1)	(1)	(3)
Intercompany debt	0	0	0
Issuance of intercompany loans	0
Proceeds from issuance of collateralized loan obligations	482
Repayment of warehouse loans and equity	306
Contributions from noncontrolling interests to investment vehicles	0
Distributions to redeemable noncontrolling interests from investment vehicles	0
Other	0	0	0
Net cash flows provided by (used in) financing activities	(660)	(780)	(803)
Effect of foreign exchange rate changes	3	(4)	5
Increase (decrease) in cash and restricted cash	76	(6)	(17)
Cash and restricted cash at beginning of period	103	109	126
Cash and restricted cash at end of period	179	103	109
Consolidation, Eliminations [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	(1,263)	(1,133)	(1,223)
Fixed-maturity securities:
Purchases	3	100	27
Sales	0	0	0
Maturities and paydowns	0	0	0
Short-term investments with maturities of over three months:
Purchases	0	0	0
Sales	0	0	0
Maturities and paydowns	0	0	0
Net sales (purchases) of short-term investments with maturities of less than three months	0	0	0
Net proceeds from paydowns on FG VIEs’ assets	0	0	0
Net proceeds from sales of financial guaranty variable interest entities’ assets	0
Investment in subsidiaries	105	11	167
Intercompany debt	(10)	(10)	(10)
Investing Activities, Issuance of Intercompany Loans	250
Proceeds from sale of subsidiaries			(139)
Return of capital from subsidiaries	(110)	(200)	(171)
Acquisition, net of cash acquired	0		0
Other	0	0	0
Net cash flows provided by (used in) investing activities	238	(99)	(126)
Cash flows from financing activities
Return of capital	10		70
Capital contribution	(105)	(11)	(28)
Dividends paid	1,249	1,133	1,223
Repurchases of common stock	100	200	101
Net paydowns of FG VIEs’ liabilities	0	0	0
Paydown of long-term debt	(3)	(100)	(27)
Intercompany debt	10	10	10
Issuance of intercompany loans	(250)
Proceeds from issuance of collateralized loan obligations	0
Repayment of warehouse loans and equity	0
Contributions from noncontrolling interests to investment vehicles	18
Distributions to redeemable noncontrolling interests from investment vehicles	(4)
Other	0	0	0
Net cash flows provided by (used in) financing activities	1,025	1,232	1,349
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	0	0	0
Cash and restricted cash at end of period	$ 0	$ 0	$ 0