Subsidiary Information (Details 3) (USD $)	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010
Condensed financial statements
Net cash flows provided by (used in) operating activities	$ 601,000,000	$ (104,300,000)
Fixed maturity securities:
Purchases	(1,787,990,000)	(1,928,744,000)
Sales	848,416,000	835,716,000
Maturities	502,929,000	729,559,000
Sales (purchases) of short-term investments, net	182,407,000	731,216,000
Net proceeds from financial guaranty variable entities' assets	598,160,000	323,626,000
Other	13,988,000	15,718,000
Net cash flows provided by (used in) investing activities	357,900,000	707,100,000
Cash flows from financing activities
Dividends paid	(24,830,000)	(24,889,000)
Repurchases of common stock	(23,311,000)	(10,457,000)
Share activity under option and incentive plans	(2,609,000)	(2,323,000)
Net paydowns of financial guaranty variable entities' liabilities	(830,063,000)	(497,296,000)
Payment of long-term debt	(16,902,000)	(16,090,000)
Net cash flows provided by (used in) financing activities	(897,800,000)	(551,000,000)
Effect of foreign exchange rate changes	3,800,000	(1,500,000)
Increase (decrease) in cash	64,900,000	50,300,000
Cash at beginning of period	108,389,000	44,133,000
Cash at end of period	173,260,000	94,406,000
Total Shareholders' Equity Attributable to Assured Guaranty Ltd.
Condensed financial statements
Net cash flows provided by (used in) operating activities	40,600,000	7,100,000
Fixed maturity securities:
Sales (purchases) of short-term investments, net	10,200,000	30,500,000
Net cash flows provided by (used in) investing activities	10,200,000	30,500,000
Cash flows from financing activities
Dividends paid	(24,800,000)	(24,900,000)
Repurchases of common stock	(23,311,000)	(10,500,000)
Share activity under option and incentive plans	(2,700,000)	(2,200,000)
Net cash flows provided by (used in) financing activities	(50,800,000)	(37,600,000)
Increase (decrease) in cash	0
Cash at end of period	0
AGUS (Issuer)
Condensed financial statements
Net cash flows provided by (used in) operating activities	12,900,000	14,400,000
Fixed maturity securities:
Sales (purchases) of short-term investments, net	(23,600,000)	(14,500,000)
Net cash flows provided by (used in) investing activities	(23,600,000)	(14,500,000)
Cash flows from financing activities
Increase (decrease) in cash	(10,700,000)	(100,000)
Cash at beginning of period	13,000,000	100,000
Cash at end of period	2,300,000
AGMH (Issuer)
Condensed financial statements
Net cash flows provided by (used in) operating activities	(33,600,000)	(32,200,000)
Fixed maturity securities:
Purchases	(3,300,000)	(14,900,000)
Maturities	500,000	5,600,000
Sales (purchases) of short-term investments, net	(2,600,000)	16,500,000
Investment in subsidiary	25,000,000	25,000,000
Other	14,000,000
Net cash flows provided by (used in) investing activities	33,600,000	32,200,000
Other Subsidiaries
Condensed financial statements
Net cash flows provided by (used in) operating activities	655,100,000	(41,600,000)
Fixed maturity securities:
Purchases	(1,784,700,000)	(1,913,800,000)
Sales	848,400,000	835,700,000
Maturities	502,400,000	724,000,000
Sales (purchases) of short-term investments, net	198,400,000	698,700,000
Net proceeds from financial guaranty variable entities' assets	598,200,000	323,600,000
Other		15,700,000
Net cash flows provided by (used in) investing activities	362,700,000	683,900,000
Cash flows from financing activities
Return of capital	(25,000,000)	(25,000,000)
Dividends paid	(74,000,000)	(52,000,000)
Net paydowns of financial guaranty variable entities' liabilities	(830,100,000)	(497,300,000)
Payment of long-term debt	(16,900,000)	(16,100,000)
Net cash flows provided by (used in) financing activities	(946,000,000)	(590,400,000)
Effect of foreign exchange rate changes	3,800,000	(1,500,000)
Increase (decrease) in cash	75,600,000	50,400,000
Cash at beginning of period	95,400,000	44,000,000
Cash at end of period	171,000,000	94,400,000
Consolidating Adjustments
Condensed financial statements
Net cash flows provided by (used in) operating activities	(74,000,000)	(52,000,000)
Fixed maturity securities:
Investment in subsidiary	(25,000,000)	(25,000,000)
Net cash flows provided by (used in) investing activities	(25,000,000)	(25,000,000)
Cash flows from financing activities
Return of capital	25,000,000	25,000,000
Dividends paid	74,000,000	52,000,000
Net cash flows provided by (used in) financing activities	$ 99,000,000	$ 77,000,000