Subsidiary Information - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities				$ (365)	$ 352
Fixed-maturity securities:
Purchases				(688)	(1,478)
Sales	$ 405		$ 316	1,306	908
Maturities and paydowns				664	746
Short-term investments with maturities of over three months:
Purchases				(216)	(148)
Sales				2	1
Maturities and paydowns				206	136
Net sales (purchases) of short-term investments with original maturities of less than three months				(404)	(80)
Net proceeds from paydowns on FG VIEs’ assets				119	90
Net proceeds from sales of FG VIEs’ assets				51	0
Proceeds from stock redemption and return of capital from subsidiaries				0	0
Other				30	19
Net cash flows provided by (used in) investing activities				1,070	194
Cash flows from financing activities
Return of capital				0
Dividends paid				(56)	(55)
Repurchases of common stock				(340)	(380)
Net paydowns of financial guaranty variable interest entities’ liabilities				(162)	(90)
Paydown of long-term debt				(4)	(73)
Other				(16)	(8)
Net cash flows provided by (used in) financing activities				(578)	(606)
Effect of foreign exchange rate changes				(2)	(2)
Increase (decrease) in cash				125	(62)
Cash and restricted cash at beginning of period		$ 82		104	144	$ 144
Cash and restricted cash at end of period	229	104	82	229	82	104
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				408	454
Fixed-maturity securities:
Purchases				0	0
Sales				0	0
Maturities and paydowns				0	0
Short-term investments with maturities of over three months:
Purchases				0	0
Sales				0	0
Maturities and paydowns				0	0
Net sales (purchases) of short-term investments with original maturities of less than three months				4	(11)
Net proceeds from paydowns on FG VIEs’ assets				0	0
Net proceeds from sales of FG VIEs’ assets				0
Proceeds from stock redemption and return of capital from subsidiaries				0	0
Other				0	0
Net cash flows provided by (used in) investing activities				4	(11)
Cash flows from financing activities
Return of capital				0
Dividends paid				(56)	(55)
Repurchases of common stock				(340)	(380)
Net paydowns of financial guaranty variable interest entities’ liabilities				0	0
Paydown of long-term debt				0	0
Other				(16)	(8)
Net cash flows provided by (used in) financing activities				(412)	(443)
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				217	101
Fixed-maturity securities:
Purchases				(3)	(76)
Sales				0	31
Maturities and paydowns				4	28
Short-term investments with maturities of over three months:
Purchases				0	0
Sales				0	0
Maturities and paydowns				12	0
Net sales (purchases) of short-term investments with original maturities of less than three months				(90)	60
Net proceeds from paydowns on FG VIEs’ assets				0	0
Net proceeds from sales of FG VIEs’ assets				0
Proceeds from stock redemption and return of capital from subsidiaries				100	200
Other				0	(15)
Net cash flows provided by (used in) investing activities				23	228
Cash flows from financing activities
Return of capital				0
Dividends paid				(237)	(362)
Repurchases of common stock				0	0
Net paydowns of financial guaranty variable interest entities’ liabilities				0	0
Paydown of long-term debt				0	0
Other				0	0
Net cash flows provided by (used in) financing activities				(237)	(362)
Effect of foreign exchange rate changes				0	0
Increase (decrease) in cash				3	(33)
Cash and restricted cash at beginning of period		0		1	33	33
Cash and restricted cash at end of period	4	1	0	4	0	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				157	126
Fixed-maturity securities:
Purchases				0	(12)
Sales				0	8
Maturities and paydowns				7	0
Short-term investments with maturities of over three months:
Purchases				0	0
Sales				0	0
Maturities and paydowns				0	0
Net sales (purchases) of short-term investments with original maturities of less than three months				(3)	(74)
Net proceeds from paydowns on FG VIEs’ assets				0	0
Net proceeds from sales of FG VIEs’ assets				0
Proceeds from stock redemption and return of capital from subsidiaries				0	0
Other				0	0
Net cash flows provided by (used in) investing activities				4	(78)
Cash flows from financing activities
Return of capital				0
Dividends paid				(156)	(50)
Repurchases of common stock				0	0
Net paydowns of financial guaranty variable interest entities’ liabilities				0	0
Paydown of long-term debt				0	0
Other				0	0
Net cash flows provided by (used in) financing activities				(156)	(50)
Effect of foreign exchange rate changes				0	0
Increase (decrease) in cash				5	(2)
Cash and restricted cash at beginning of period		0		0	2	2
Cash and restricted cash at end of period	5	0	0	5	0	0
Reportable Legal Entities [Member] | Other Entities [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities				(280)	448
Fixed-maturity securities:
Purchases				(688)	(1,462)
Sales				1,306	869
Maturities and paydowns				653	718
Short-term investments with maturities of over three months:
Purchases				(216)	(148)
Sales				2	1
Maturities and paydowns				194	136
Net sales (purchases) of short-term investments with original maturities of less than three months				(315)	(55)
Net proceeds from paydowns on FG VIEs’ assets				119	90
Net proceeds from sales of FG VIEs’ assets				51
Proceeds from stock redemption and return of capital from subsidiaries				10	0
Other				30	34
Net cash flows provided by (used in) investing activities				1,146	183
Cash flows from financing activities
Return of capital				(10)
Dividends paid				(474)	(365)
Repurchases of common stock				(100)	(200)
Net paydowns of financial guaranty variable interest entities’ liabilities				(162)	(90)
Paydown of long-term debt				(1)	(1)
Other				0	0
Net cash flows provided by (used in) financing activities				(747)	(656)
Effect of foreign exchange rate changes				(2)	(2)
Increase (decrease) in cash				117	(27)
Cash and restricted cash at beginning of period		82		103	109	109
Cash and restricted cash at end of period	220	103	82	220	82	103
Consolidating Adjustments [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities				(867)	(777)
Fixed-maturity securities:
Purchases				3	72
Sales				0	0
Maturities and paydowns				0	0
Short-term investments with maturities of over three months:
Purchases				0	0
Sales				0	0
Maturities and paydowns				0	0
Net sales (purchases) of short-term investments with original maturities of less than three months				0	0
Net proceeds from paydowns on FG VIEs’ assets				0	0
Net proceeds from sales of FG VIEs’ assets				0
Proceeds from stock redemption and return of capital from subsidiaries				(110)	(200)
Other				0	0
Net cash flows provided by (used in) investing activities				(107)	(128)
Cash flows from financing activities
Return of capital				10
Dividends paid				867	777
Repurchases of common stock				100	200
Net paydowns of financial guaranty variable interest entities’ liabilities				0	0
Paydown of long-term debt				(3)	(72)
Other				0	0
Net cash flows provided by (used in) financing activities				974	905
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