Subsidiary Information - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended		12 Months Ended
	Dec. 31, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities		$ (39)	$ 347
Fixed-maturity securities:
Purchases		(1,844)	(2,031)
Sales		1,719	951
Maturities		635	557
Net sales (purchases) of short-term investments		751	89
Net proceeds from financial guaranty variable entities’ assets		114	346
Intercompany debt		0	0
Investment in subsidiary		0	0
Acquisition of Radian Asset, net of cash acquired		(800)	0
Other		59	9
Net cash flows provided by (used in) investing activities		634	(79)
Cash flows from financing activities
Return of capital		0	0
Dividends paid		(55)	(58)
Repurchases of common stock		(420)	(438)
Share activity under option and incentive plans		(2)	(1)
Net paydowns of financial guaranty variable interest entities’ liabilities		(122)	(348)
Proceeds from issuance of long-term debt		0	495
Payment of long-term debt		(3)	(18)
Intercompany debt		0	0
Net cash flows provided by (used in) financing activities		(602)	(368)
Effect of foreign exchange rate changes		(2)	(2)
Increase (decrease) in cash		(9)	(102)
Cash at beginning of period	$ 82	75	184	$ 184
Cash at end of period	75	66	82	75
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		451	762
Fixed-maturity securities:
Purchases		0	0
Sales		0	0
Maturities		0	0
Net sales (purchases) of short-term investments		26	(265)
Net proceeds from financial guaranty variable entities’ assets		0	0
Intercompany debt		0	0
Investment in subsidiary		0	0
Acquisition of Radian Asset, net of cash acquired		0
Other		0	0
Net cash flows provided by (used in) investing activities		26	(265)
Cash flows from financing activities
Return of capital		0	0
Dividends paid		(55)	(58)
Repurchases of common stock		(420)	(438)
Share activity under option and incentive plans		(2)	(1)
Net paydowns of financial guaranty variable interest entities’ liabilities		0	0
Proceeds from issuance of long-term debt			0
Payment of long-term debt		0	0
Intercompany debt		0	0
Net cash flows provided by (used in) financing activities		(477)	(497)
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
Reportable Legal Entities [Member] | AGUS (Issuer) [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities		209	170
Fixed-maturity securities:
Purchases		(72)	(356)
Sales		170	399
Maturities		8	4
Net sales (purchases) of short-term investments		42	(74)
Net proceeds from financial guaranty variable entities’ assets		0	0
Intercompany debt		0	0
Investment in subsidiary		0	0
Acquisition of Radian Asset, net of cash acquired		0
Other		(5)	0
Net cash flows provided by (used in) investing activities		143	(27)
Cash flows from financing activities
Return of capital		0	0
Dividends paid		(351)	(700)
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
Proceeds from issuance of long-term debt			495
Payment of long-term debt		0	0
Intercompany debt		0	0
Net cash flows provided by (used in) financing activities		(351)	(205)
Effect of foreign exchange rate changes		0	0
Increase (decrease) in cash		1	(62)
Cash at beginning of period	5	0	67	67
Cash at end of period	0	1	5	0
Reportable Legal Entities [Member] | AGMH (Issuer) [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities		134	84
Fixed-maturity securities:
Purchases		(8)	(7)
Sales		27	8
Maturities		0	1
Net sales (purchases) of short-term investments		4	14
Net proceeds from financial guaranty variable entities’ assets		0	0
Intercompany debt		0	0
Investment in subsidiary		25	50
Acquisition of Radian Asset, net of cash acquired		0
Other		0	0
Net cash flows provided by (used in) investing activities		48	66
Cash flows from financing activities
Return of capital		0	0
Dividends paid		(182)	(150)
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
Proceeds from issuance of long-term debt			0
Payment of long-term debt		0	0
Intercompany debt		0	0
Net cash flows provided by (used in) financing activities		(182)	(150)
Effect of foreign exchange rate changes		0	0
Increase (decrease) in cash		0	0
Cash at beginning of period	0	4	0	0
Cash at end of period	4	4	0	4
Reportable Legal Entities [Member] | Other Entities [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities		99	408
Fixed-maturity securities:
Purchases		(1,830)	(1,668)
Sales		1,522	544
Maturities		627	552
Net sales (purchases) of short-term investments		679	414
Net proceeds from financial guaranty variable entities’ assets		114	346
Intercompany debt		0	0
Investment in subsidiary		0	0
Acquisition of Radian Asset, net of cash acquired		(800)
Other		64	9
Net cash flows provided by (used in) investing activities		376	197
Cash flows from financing activities
Return of capital		(25)	(50)
Dividends paid		(333)	(227)
Repurchases of common stock		0	0
Share activity under option and incentive plans		0	0
Net paydowns of financial guaranty variable interest entities’ liabilities		(122)	(348)
Proceeds from issuance of long-term debt			0
Payment of long-term debt		(3)	(18)
Intercompany debt		0	0
Net cash flows provided by (used in) financing activities		(483)	(643)
Effect of foreign exchange rate changes		(2)	(2)
Increase (decrease) in cash		(10)	(40)
Cash at beginning of period	77	71	117	117
Cash at end of period	71	61	77	71
Consolidating Adjustments [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash flows provided by (used in) operating activities		(932)	(1,077)
Fixed-maturity securities:
Purchases		66	0
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
Intercompany debt		0	0
Investment in subsidiary		(25)	(50)
Acquisition of Radian Asset, net of cash acquired		0
Other		0	0
Net cash flows provided by (used in) investing activities		41	(50)
Cash flows from financing activities
Return of capital		25	50
Dividends paid		866	1,077
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
Proceeds from issuance of long-term debt			0
Payment of long-term debt		0	0
Intercompany debt		0	0
Net cash flows provided by (used in) financing activities		891	1,127
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