Subsidiary Information - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	$ (332)	$ 27
Fixed-maturity securities:
Purchases	(196)	(411)
Sales	471	409
Maturities	177	225
Short-term investments with maturities of over three months:
Purchases	(107)	(47)
Sales	2	0
Maturities	67	45
Net sales (purchases) of short-term investments with original maturities of less than three months	25	(114)
Net proceeds from FG VIEs’ assets	26	33
Proceeds from stock redemption and return of capital from subsidiaries		0
Other		(14)
Other	27	(14)
Net cash flows provided by (used in) investing activities	492	126
Cash flows from financing activities
Dividends paid	(20)	(18)
Repurchases of common stock	(80)	(100)
Repurchases of common stock to pay withholding taxes	(15)	(12)
Net paydowns of financial guaranty variable interest entities’ liabilities	(25)	(33)
Paydown of long-term debt	(3)	(19)
Proceeds from option exercises	1	1
Net cash flows provided by (used in) financing activities	(142)	(181)
Effect of foreign exchange rate changes	1	1
Increase (decrease) in cash	19	(27)
Cash and restricted cash at beginning of period	104	144	$ 144
Cash and restricted cash at end of period	123	117	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	112	128
Fixed-maturity securities:
Purchases	0	0
Sales	0	0
Maturities	0	0
Short-term investments with maturities of over three months:
Purchases	0	0
Sales	0	0
Maturities	0	0
Net sales (purchases) of short-term investments with original maturities of less than three months	2	1
Net proceeds from FG VIEs’ assets	0	0
Proceeds from stock redemption and return of capital from subsidiaries		0
Other		0
Other	0
Net cash flows provided by (used in) investing activities	2	1
Cash flows from financing activities
Dividends paid	(20)	(18)
Repurchases of common stock	(80)	(100)
Repurchases of common stock to pay withholding taxes	(15)	(12)
Net paydowns of financial guaranty variable interest entities’ liabilities	0	0
Paydown of long-term debt	0	0
Proceeds from option exercises	1	1
Net cash flows provided by (used in) financing activities	(114)	(129)
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	87	83
Fixed-maturity securities:
Purchases	(3)	(18)
Sales	0	11
Maturities	0	0
Short-term investments with maturities of over three months:
Purchases	0	0
Sales	0	0
Maturities	8	0
Net sales (purchases) of short-term investments with original maturities of less than three months	(33)	(217)
Net proceeds from FG VIEs’ assets	0	0
Proceeds from stock redemption and return of capital from subsidiaries		200
Other		(13)
Other	0
Net cash flows provided by (used in) investing activities	(28)	(37)
Cash flows from financing activities
Dividends paid	(60)	(78)
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	(60)	(78)
Effect of foreign exchange rate changes	0	0
Increase (decrease) in cash	(1)	(32)
Cash and restricted cash at beginning of period	1	33	33
Cash and restricted cash at end of period	0	1	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	68	63
Fixed-maturity securities:
Purchases	0	(12)
Sales	0	2
Maturities	1	0
Short-term investments with maturities of over three months:
Purchases	0	0
Sales	0	0
Maturities	0	0
Net sales (purchases) of short-term investments with original maturities of less than three months	(20)	(4)
Net proceeds from FG VIEs’ assets	0	0
Proceeds from stock redemption and return of capital from subsidiaries		0
Other		0
Other	0
Net cash flows provided by (used in) investing activities	(19)	(14)
Cash flows from financing activities
Dividends paid	(47)	(50)
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	(47)	(50)
Effect of foreign exchange rate changes	0	0
Increase (decrease) in cash	2	(1)
Cash and restricted cash at beginning of period	0	2	2
Cash and restricted cash at end of period	2	1	0
Reportable Legal Entities [Member] | Other Entities [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	(334)	46
Fixed-maturity securities:
Purchases	(196)	(400)
Sales	471	396
Maturities	176	225
Short-term investments with maturities of over three months:
Purchases	(107)	(47)
Sales	2	0
Maturities	59	45
Net sales (purchases) of short-term investments with original maturities of less than three months	76	106
Net proceeds from FG VIEs’ assets	26	33
Proceeds from stock redemption and return of capital from subsidiaries		0
Other		(1)
Other	27
Net cash flows provided by (used in) investing activities	534	357
Cash flows from financing activities
Dividends paid	(158)	(165)
Repurchases of common stock	0	0
Repurchases of common stock to pay withholding taxes	0	(200)
Net paydowns of financial guaranty variable interest entities’ liabilities	(25)	(33)
Paydown of long-term debt	0	0
Proceeds from option exercises	0	0
Net cash flows provided by (used in) financing activities	(183)	(398)
Effect of foreign exchange rate changes	1	1
Increase (decrease) in cash	18	6
Cash and restricted cash at beginning of period	103	109	109
Cash and restricted cash at end of period	121	115	103
Consolidating Adjustments [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities	(265)	(293)
Fixed-maturity securities:
Purchases	3	19
Sales	0	0
Maturities	0	0
Short-term investments with maturities of over three months:
Purchases	0	0
Sales	0	0
Maturities	0	0
Net sales (purchases) of short-term investments with original maturities of less than three months	0	0
Net proceeds from FG VIEs’ assets	0	0
Proceeds from stock redemption and return of capital from subsidiaries		(200)
Other		0
Other	0
Net cash flows provided by (used in) investing activities	3	(181)
Cash flows from financing activities
Dividends paid	265	293
Repurchases of common stock	0	0
Repurchases of common stock to pay withholding taxes	0	200
Net paydowns of financial guaranty variable interest entities’ liabilities	0	0
Paydown of long-term debt	(3)	(19)
Proceeds from option exercises	0	0
Net cash flows provided by (used in) financing activities	262	474
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