Subsidiary Information - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended		12 Months Ended
	Dec. 31, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities		$ (190)	$ (39)
Fixed-maturity securities:
Purchases		(1,028)	(1,844)
Sales		877	1,719
Maturities		861	635
Net sales (purchases) of short-term investments		80	751
Net proceeds from financial guaranty variable entities’ assets		590	114
Investment in subsidiary		0	0
Acquisition of Radian Asset, net of cash acquired		0	(800)
Cash restricted for CIFG acquisition		(435)
Other		(12)	59
Net cash flows provided by (used in) investing activities		933	634
Cash flows from financing activities
Return of capital		0	0
Dividends paid		(52)	(55)
Repurchases of common stock		(190)	(420)
Share activity under option and incentive plans		4	(2)
Net paydowns of financial guaranty variable interest entities’ liabilities		(567)	(122)
Payment of long-term debt		(2)	(3)
Net cash flows provided by (used in) financing activities		(807)	(602)
Effect of foreign exchange rate changes		(4)	(2)
Increase (decrease) in cash		(68)	(9)
Cash at beginning of period	$ 66	166	75	$ 75
Cash at end of period	166	98	66	166
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		287	451
Fixed-maturity securities:
Purchases		(4)	0
Sales		4	0
Maturities		0	0
Net sales (purchases) of short-term investments		(49)	26
Net proceeds from financial guaranty variable entities’ assets		0	0
Investment in subsidiary		0	0
Acquisition of Radian Asset, net of cash acquired			0
Cash restricted for CIFG acquisition		0
Other		0	0
Net cash flows provided by (used in) investing activities		(49)	26
Cash flows from financing activities
Return of capital		0	0
Dividends paid		(52)	(55)
Repurchases of common stock		(190)	(420)
Share activity under option and incentive plans		5	(2)
Net paydowns of financial guaranty variable interest entities’ liabilities		0	0
Payment of long-term debt		0	0
Net cash flows provided by (used in) financing activities		(237)	(477)
Effect of foreign exchange rate changes		0	0
Increase (decrease) in cash		1	0
Cash at beginning of period	0	0	0	0
Cash at end of period	0	1	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		171	209
Fixed-maturity securities:
Purchases		(44)	(72)
Sales		0	170
Maturities		23	8
Net sales (purchases) of short-term investments		(3)	42
Net proceeds from financial guaranty variable entities’ assets		0	0
Investment in subsidiary		0	0
Acquisition of Radian Asset, net of cash acquired			0
Cash restricted for CIFG acquisition		0
Other		7	(5)
Net cash flows provided by (used in) investing activities		(17)	143
Cash flows from financing activities
Return of capital		0	0
Dividends paid		(223)	(351)
Repurchases of common stock		0	0
Share activity under option and incentive plans		0	0
Net paydowns of financial guaranty variable interest entities’ liabilities		0	0
Payment of long-term debt		0	0
Net cash flows provided by (used in) financing activities		(223)	(351)
Effect of foreign exchange rate changes		0	0
Increase (decrease) in cash		(69)	1
Cash at beginning of period	1	95	0	0
Cash at end of period	95	26	1	95
Reportable Legal Entities [Member] | AGMH (Issuer) [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities		159	134
Fixed-maturity securities:
Purchases		(10)	(8)
Sales		12	27
Maturities		0	0
Net sales (purchases) of short-term investments		(1)	4
Net proceeds from financial guaranty variable entities’ assets		0	0
Investment in subsidiary		0	25
Acquisition of Radian Asset, net of cash acquired			0
Cash restricted for CIFG acquisition		0
Other		0	0
Net cash flows provided by (used in) investing activities		1	48
Cash flows from financing activities
Return of capital		0	0
Dividends paid		(158)	(182)
Repurchases of common stock		0	0
Share activity under option and incentive plans		0	0
Net paydowns of financial guaranty variable interest entities’ liabilities		0	0
Payment of long-term debt		0	0
Net cash flows provided by (used in) financing activities		(158)	(182)
Effect of foreign exchange rate changes		0	0
Increase (decrease) in cash		2	0
Cash at beginning of period	4	8	4	4
Cash at end of period	8	10	4	8
Reportable Legal Entities [Member] | Other Entities [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities		3	99
Fixed-maturity securities:
Purchases		(970)	(1,830)
Sales		861	1,522
Maturities		838	627
Net sales (purchases) of short-term investments		133	679
Net proceeds from financial guaranty variable entities’ assets		590	114
Investment in subsidiary		4	0
Acquisition of Radian Asset, net of cash acquired			(800)
Cash restricted for CIFG acquisition		(442)
Other		(12)	64
Net cash flows provided by (used in) investing activities		1,002	376
Cash flows from financing activities
Return of capital		(4)	(25)
Dividends paid		(429)	(333)
Repurchases of common stock		0	0
Share activity under option and incentive plans		(1)	0
Net paydowns of financial guaranty variable interest entities’ liabilities		(567)	(122)
Payment of long-term debt		(2)	(3)
Net cash flows provided by (used in) financing activities		(1,003)	(483)
Effect of foreign exchange rate changes		(4)	(2)
Increase (decrease) in cash		(2)	(10)
Cash at beginning of period	61	63	71	71
Cash at end of period	63	61	61	63
Consolidating Adjustments [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities		(810)	(932)
Fixed-maturity securities:
Purchases		0	66
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		(4)	(25)
Acquisition of Radian Asset, net of cash acquired			0
Cash restricted for CIFG acquisition		7
Other		(7)	0
Net cash flows provided by (used in) investing activities		(4)	41
Cash flows from financing activities
Return of capital		4	25
Dividends paid		810	866
Repurchases of common stock		0	0
Share activity under option and incentive plans		0	0
Net paydowns of financial guaranty variable interest entities’ liabilities		0	0
Payment of long-term debt		0	0
Net cash flows provided by (used in) financing activities		814	891
Effect of foreign exchange rate changes		0	0
Increase (decrease) in cash		0	0
Cash at beginning of period	0	0	0	0
Cash at end of period	$ 0	$ 0	$ 0	$ 0