Subsidiary Information (Details 3) (USD $)	1 Months Ended		12 Months Ended
	Aug. 31, 2011	Jun. 30, 2009	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Condensed financial statements
Net cash flows provided by (used in) operating activities			$ 675,619,000	$ 129,189,000	$ 279,170,000
Fixed maturity securities:
Purchases			(2,308,078,000)	(2,461,692,000)	(2,287,668,000)
Sales			1,107,473,000	1,063,581,000	1,519,300,000
Maturities			662,970,000	994,360,000	217,895,000
Sales (purchases) of short-term investments, net			319,992,000	613,267,000	(397,100,000)
Net proceeds from financial guaranty variable entities' assets			760,374,000	423,997,000
Acquisition of AGMH					(458,998,000)
Other			18,618,000	19,786,000	9,350,000
Net cash flows provided by (used in) investing activities			561,349,000	653,299,000	(1,397,221,000)
Cash flows from financing activities
Net proceeds from issuance of common stock and equity units		616,500,000			1,190,100,000
Dividends paid			(33,035,000)	(33,190,000)	(22,332,000)
Repurchases of common stock	(23,311,000)		(23,311,000)	(10,457,000)	(3,676,000)
Share activity under option and incentive plans			(1,192,000)	(2,062,000)	(667,000)
Net paydowns of financial guaranty variable entities' liabilities			(1,053,280,000)	(650,862,000)
Payment of long-term debt			(22,137,000)	(20,891,000)	(14,823,000)
Net cash flows provided by (used in) financing activities			(1,132,955,000)	(717,434,000)	1,148,554,000
Effect of exchange rate changes			2,142,000	(798,000)	1,325,000
Increase in cash			106,155,000	64,256,000	31,828,000
Cash at beginning of year			108,389,000	44,133,000	12,305,000
Cash at end of year			214,544,000	108,389,000	44,133,000
Total Shareholders' Equity Attributable to Assured Guaranty Ltd.
Condensed financial statements
Net cash flows provided by (used in) operating activities			67,900,000	15,700,000	19,600,000
Fixed maturity securities:
Sales (purchases) of short-term investments, net			(10,400,000)	30,000,000	(52,300,000)
Capital contribution to subsidiary					(962,900,000)
Net cash flows provided by (used in) investing activities			(10,400,000)	30,000,000	(1,015,200,000)
Cash flows from financing activities
Net proceeds from issuance of common stock and equity units					1,022,800,000
Dividends paid			(33,000,000)	(33,200,000)	(22,800,000)
Repurchases of common stock			(23,300,000)	(10,500,000)	(3,700,000)
Share activity under option and incentive plans			(1,200,000)	(2,000,000)	(700,000)
Net cash flows provided by (used in) financing activities			(57,500,000)	(45,700,000)	995,600,000
AGUS (Issuer)
Condensed financial statements
Net cash flows provided by (used in) operating activities			83,800,000	11,800,000	(23,900,000)
Fixed maturity securities:
Sales (purchases) of short-term investments, net			(25,100,000)	1,100,000	(3,500,000)
Capital contribution to subsidiary					(556,700,000)
Acquisition of AGMH					(546,000,000)
Net cash flows provided by (used in) investing activities			(25,100,000)	1,100,000	(1,106,200,000)
Cash flows from financing activities
Net proceeds from issuance of common stock and equity units					167,300,000
Capital contribution from parent					962,900,000
Net cash flows provided by (used in) financing activities					1,130,200,000
Increase in cash			58,700,000	12,900,000	100,000
Cash at beginning of year			13,000,000	100,000
Cash at end of year			71,700,000	13,000,000	100,000
AGMH
Condensed financial statements
Net cash flows provided by (used in) operating activities			(36,000,000)	(49,000,000)	(24,700,000)
Fixed maturity securities:
Purchases			(14,200,000)	(15,000,000)	(1,700,000)
Sales				4,500,000
Maturities			800,000	6,400,000	1,600,000
Sales (purchases) of short-term investments, net			(600,000)	3,100,000	(300,000)
Investment in subsidiary			50,000,000	50,000,000	25,000,000
Net cash flows provided by (used in) investing activities			36,000,000	49,000,000	24,600,000
Cash flows from financing activities
Increase in cash					(100,000)
Cash at beginning of year					100,000
Other Subsidiaries
Condensed financial statements
Net cash flows provided by (used in) operating activities			675,900,000	224,700,000	355,800,000
Fixed maturity securities:
Purchases			(2,293,900,000)	(2,446,700,000)	(2,286,000,000)
Sales			1,107,500,000	1,059,100,000	1,519,300,000
Maturities			662,100,000	988,000,000	216,300,000
Sales (purchases) of short-term investments, net			356,100,000	579,100,000	(341,000,000)
Net proceeds from financial guaranty variable entities' assets			760,400,000	424,000,000
Capital contribution to subsidiary					(512,000,000)
Acquisition of AGMH					87,000,000
Other			18,600,000	19,700,000	9,400,000
Net cash flows provided by (used in) investing activities			610,800,000	623,200,000	(1,307,000,000)
Cash flows from financing activities
Capital contribution from parent					1,068,700,000
Return of capital			(50,000,000)	(50,000,000)	(25,000,000)
Dividends paid			(116,000,000)	(74,000,000)	(47,100,000)
Net paydowns of financial guaranty variable entities' liabilities			(1,053,300,000)	(650,800,000)
Payment of long-term debt			(22,100,000)	(20,900,000)	(14,800,000)
Net cash flows provided by (used in) financing activities			(1,241,400,000)	(795,700,000)	981,800,000
Effect of exchange rate changes			2,100,000	(800,000)	1,200,000
Increase in cash			47,400,000	51,400,000	31,800,000
Cash at beginning of year			95,400,000	44,000,000	12,200,000
Cash at end of year			142,800,000	95,400,000	44,000,000
Consolidating Adjustments
Condensed financial statements
Net cash flows provided by (used in) operating activities			(116,000,000)	(74,000,000)	(47,600,000)
Fixed maturity securities:
Capital contribution to subsidiary					2,031,600,000
Investment in subsidiary			(50,000,000)	(50,000,000)	(25,000,000)
Net cash flows provided by (used in) investing activities			(50,000,000)	(50,000,000)	2,006,600,000
Cash flows from financing activities
Capital contribution from parent					(2,031,600,000)
Return of capital			50,000,000	50,000,000	25,000,000
Dividends paid			116,000,000	74,000,000	47,600,000
Net cash flows provided by (used in) financing activities			$ 166,000,000	$ 124,000,000	$ (1,959,000,000)