Subsidiary Information (Details 3) (USD $) In Millions, unless otherwise specified	1 Months Ended	9 Months Ended
	Aug. 31, 2011	Sep. 30, 2012	Sep. 30, 2011
Condensed financial statements
Net cash flows provided by (used in) operating activities		$ (223)	$ 601
Fixed maturity securities:
Purchases		(1,322)	(1,788)
Sales		683	849
Maturities		758	503
Sales (purchases) of short-term investments, net		282	182
Net proceeds from financial guaranty variable entities' assets		(407)	(598)
Acquisition of MIAC, net of cash acquired		(91)	0
Investing Activities, Intercompany Debt		0
Investing Activities, Investment in Subsidiary		0	0
Payments for (Proceeds from) Other Investing Activities		85	14
Net Cash Provided by (Used in) Investing Activities, Continuing Operations		802	358
Net cash flows used in investing activities
Proceeds from issuances of common stock		173	0
Return of capital		0	0
Dividends paid		(51)	(25)
Repurchases of common stock		(24)	(23)
Share activity under option and incentive plans		(3)	(3)
Net paydowns of financial guaranty variable entities' liabilities		(553)	(830)
Payment of long-term debt		(204)	(17)
Financing Activities, Intercompany Debt		0
Net cash flows provided by (used in) financing activities		(662)	(898)
Effect of exchange rate changes		1	4
Increase (decrease) in cash		(82)	65
Cash at beginning of period		215	108
Cash at end of period		133	173
Assured Guaranty Ltd. (Parent)
Condensed financial statements
Net cash flows provided by (used in) operating activities		98	41
Fixed maturity securities:
Purchases		(160)	0
Sales		0	0
Maturities		1	0
Sales (purchases) of short-term investments, net		(34)	10
Acquisition of MIAC, net of cash acquired		0
Investing Activities, Intercompany Debt		0
Investing Activities, Investment in Subsidiary		0	0
Payments for (Proceeds from) Other Investing Activities		0	0
Net cash flows provided by (used in) investing activities		(193)	10
Net cash flows used in investing activities
Proceeds from issuances of common stock		173
Return of capital		0	0
Dividends paid		(51)	(25)
Repurchases of common stock	(23)	(24)	(23)
Share activity under option and incentive plans		(3)	(3)
Payment of long-term debt		0	0
Financing Activities, Intercompany Debt		0
Net cash flows provided by (used in) financing activities		95	(51)
Effect of exchange rate changes		0	0
Increase (decrease) in cash		0	0
Cash at beginning of period		0	0
Cash at end of period		0	0
AGUS (Issuer)
Condensed financial statements
Net cash flows provided by (used in) operating activities		23	13
Fixed maturity securities:
Purchases		(1)	0
Sales		0	0
Maturities		0	0
Sales (purchases) of short-term investments, net		27	(24)
Acquisition of MIAC, net of cash acquired		(91)
Investing Activities, Intercompany Debt		0
Investing Activities, Investment in Subsidiary		0	0
Payments for (Proceeds from) Other Investing Activities		0	0
Net cash flows provided by (used in) investing activities		(65)	(24)
Net cash flows used in investing activities
Proceeds from issuances of common stock		0
Return of capital		0	0
Dividends paid		0	0
Repurchases of common stock		0	0
Payment of long-term debt		(173)	0
Financing Activities, Intercompany Debt		173
Net cash flows provided by (used in) financing activities		0	0
Effect of exchange rate changes		0	0
Increase (decrease) in cash		(42)	(11)
Cash at beginning of period		72	13
Cash at end of period		30	2
AGMH (Issuer)
Condensed financial statements
Net cash flows provided by (used in) operating activities		14	(20)
Fixed maturity securities:
Purchases		(11)	(3)
Sales		1	0
Maturities		3	1
Sales (purchases) of short-term investments, net		23	(3)
Acquisition of MIAC, net of cash acquired		0
Investing Activities, Intercompany Debt		0
Investing Activities, Investment in Subsidiary		50	25
Payments for (Proceeds from) Other Investing Activities		0	0
Net cash flows provided by (used in) investing activities		66	20
Net cash flows used in investing activities
Proceeds from issuances of common stock		0
Return of capital		0	0
Dividends paid		(80)	0
Repurchases of common stock		0	0
Payment of long-term debt		0	0
Financing Activities, Intercompany Debt		0
Net cash flows provided by (used in) financing activities		(80)	0
Effect of exchange rate changes		0	0
Increase (decrease) in cash		0	0
Cash at beginning of period		0	0
Cash at end of period		0	0
Other Subsidiaries
Condensed financial statements
Net cash flows provided by (used in) operating activities		(82)	641
Fixed maturity securities:
Purchases		(1,150)	(1,785)
Sales		682	849
Maturities		754	502
Sales (purchases) of short-term investments, net		266	199
Net proceeds from financial guaranty variable entities' assets		(407)	(598)
Acquisition of MIAC, net of cash acquired		0
Investing Activities, Intercompany Debt		(173)
Investing Activities, Investment in Subsidiary		0	0
Payments for (Proceeds from) Other Investing Activities		85	14
Net cash flows provided by (used in) investing activities		871	377
Net cash flows used in investing activities
Proceeds from issuances of common stock		0
Return of capital		(50)	(25)
Dividends paid		(196)	(74)
Repurchases of common stock		0	0
Net paydowns of financial guaranty variable entities' liabilities		(553)	(830)
Payment of long-term debt		(31)	(17)
Financing Activities, Intercompany Debt		0
Net cash flows provided by (used in) financing activities		(830)	(946)
Effect of exchange rate changes		1	4
Increase (decrease) in cash		(40)	76
Cash at beginning of period		143	95
Cash at end of period		103	171
Consolidating Adjustments
Condensed financial statements
Net cash flows provided by (used in) operating activities		(276)	(74)
Fixed maturity securities:
Purchases		0	0
Sales		0	0
Maturities		0	0
Sales (purchases) of short-term investments, net		0	0
Net proceeds from financial guaranty variable entities' assets		0	0
Acquisition of MIAC, net of cash acquired		0
Investing Activities, Intercompany Debt		173
Investing Activities, Investment in Subsidiary		(50)	(25)
Payments for (Proceeds from) Other Investing Activities		0	0
Net cash flows provided by (used in) investing activities		123	(25)
Net cash flows used in investing activities
Proceeds from issuances of common stock		0
Return of capital		50	25
Dividends paid		276	74
Repurchases of common stock		0	0
Payment of long-term debt		0	0
Financing Activities, Intercompany Debt		(173)
Net cash flows provided by (used in) financing activities		153	99
Effect of exchange rate changes		0	0
Increase (decrease) in cash		0	0
Cash at beginning of period		0	0
Cash at end of period		$ 0	$ 0