Subsidiary Information (Details 3) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Condensed financial statements
Net cash flows provided by (used in) operating activities	$ (14)	$ 75
Fixed maturity securities:
Purchases	(510)	(383)
Sales	183	189
Maturities	283	253
Sales (purchases) of short-term investments, net	88	(143)
Net proceeds from paydowns on financial guaranty variable interest entities' assets	138	138
Investing Activities, Investment in Subsidiary	0
Other	55	53
Net cash flows provided by (used in) investing activities	237	107
Net cash flows used in investing activities
Return of capital	0
Dividends paid	(19)	(16)
Repurchases of common stock	39	0
Share activity under option and incentive plans	(2)	(3)
Net paydowns of financial guaranty variable entities' liabilities	(167)	(193)
Payment of long-term debt	(6)	(5)
Net cash flows provided by (used in) financing activities	(233)	(217)
Effect of exchange rate changes	(3)	2
Increase (decrease) in cash	(13)	(33)
Cash at beginning of period	138	215
Cash at end of period	125	182
Assured Guaranty Ltd. (Parent)
Condensed financial statements
Net cash flows provided by (used in) operating activities	36	29
Fixed maturity securities:
Purchases	0	0
Sales	40	0
Maturities	18	0
Sales (purchases) of short-term investments, net	(34)	(10)
Net proceeds from paydowns on financial guaranty variable interest entities' assets		0
Investing Activities, Investment in Subsidiary	0
Other	0	0
Net cash flows provided by (used in) investing activities	24	(10)
Net cash flows used in investing activities
Return of capital	0
Dividends paid	(19)	(16)
Repurchases of common stock	39
Share activity under option and incentive plans	(2)	(3)
Net paydowns of financial guaranty variable entities' liabilities	0	0
Payment of long-term debt	0	0
Net cash flows provided by (used in) financing activities	(60)	(19)
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	1	(68)
Fixed maturity securities:
Purchases	(1)	0
Sales	0	0
Maturities	0	0
Sales (purchases) of short-term investments, net	0	27
Net proceeds from paydowns on financial guaranty variable interest entities' assets		0
Investing Activities, Investment in Subsidiary	0
Other	0	0
Net cash flows provided by (used in) investing activities	(1)	27
Net cash flows used in investing activities
Return of capital	0
Dividends paid	0	0
Repurchases of common stock	0
Share activity under option and incentive plans	0	0
Net paydowns of financial guaranty variable entities' liabilities	0	0
Payment of long-term debt	0	0
Net cash flows provided by (used in) financing activities	0	0
Effect of exchange rate changes	0	0
Increase (decrease) in cash	0	(41)
Cash at beginning of period	13	72
Cash at end of period	13	31
AGMH
Condensed financial statements
Net cash flows provided by (used in) operating activities	(6)	37
Fixed maturity securities:
Purchases	0	(11)
Sales	7	0
Maturities	1	1
Sales (purchases) of short-term investments, net	(27)	3
Net proceeds from paydowns on financial guaranty variable interest entities' assets		0
Investing Activities, Investment in Subsidiary	(25)
Other	0	0
Net cash flows provided by (used in) investing activities	6	(7)
Net cash flows used in investing activities
Return of capital	0
Dividends paid	0	(30)
Repurchases of common stock	0
Share activity under option and incentive plans	0	0
Net paydowns of financial guaranty variable entities' liabilities	0	0
Payment of long-term debt	0	0
Net cash flows provided by (used in) financing activities	0	(30)
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	(5)	182
Fixed maturity securities:
Purchases	(509)	(372)
Sales	136	189
Maturities	264	252
Sales (purchases) of short-term investments, net	149	(163)
Net proceeds from paydowns on financial guaranty variable interest entities' assets	138	138
Investing Activities, Investment in Subsidiary	0
Other	55	53
Net cash flows provided by (used in) investing activities	233	97
Net cash flows used in investing activities
Return of capital	(25)
Dividends paid	(40)	(75)
Repurchases of common stock	0
Share activity under option and incentive plans	0	0
Net paydowns of financial guaranty variable entities' liabilities	(167)	(193)
Payment of long-term debt	(6)	(5)
Net cash flows provided by (used in) financing activities	(238)	(273)
Effect of exchange rate changes	(3)	2
Increase (decrease) in cash	(13)	8
Cash at beginning of period	125	143
Cash at end of period	112	151
Consolidating Adjustments
Condensed financial statements
Net cash flows provided by (used in) operating activities	(40)	(105)
Fixed maturity securities:
Purchases	0	0
Sales	0	0
Maturities	0	0
Sales (purchases) of short-term investments, net	0	0
Net proceeds from paydowns on financial guaranty variable interest entities' assets	0	0
Investing Activities, Investment in Subsidiary	25
Other	0	0
Net cash flows provided by (used in) investing activities	(25)	0
Net cash flows used in investing activities
Return of capital	25
Dividends paid	40	105
Repurchases of common stock	0
Share activity under option and incentive plans	0	0
Net paydowns of financial guaranty variable entities' liabilities	0	0
Payment of long-term debt	0	0
Net cash flows provided by (used in) financing activities	65	105
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