Quarterly Financial Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	1 Months Ended	3 Months Ended								12 Months Ended
	Feb. 29, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Revenues
Net earned premiums		$ 225,000	$ 211,100	$ 230,000	$ 254,000	$ 286,300	$ 288,700	$ 297,000	$ 314,700	$ 920,055	$ 1,186,705	$ 930,429
Net investment income		100,300	93,500	101,100	96,100	93,900	85,600	90,900	84,300	391,017	354,703	259,222
Net realized investment gains (losses)		(4,600)	(11,100)	(5,100)	2,800	(600)	(2,400)	(8,400)	9,400	(17,992)	(1,994)	(32,662)
Net change in fair value of credit derivatives		(295,200)	1,155,900	(64,800)	(236,200)	(129,900)	(224,000)	73,500	278,800	559,738	(1,611)	(174,252)
Fair value gain (loss) on committed capital securities		31,600	2,400	600	500	3,400	(5,500)	12,600	(1,300)	35,130	9,195	(122,940)
Net change in fair value of FG VIEs		21,900	(99,200)	(174,300)	119,600	(408,600)	171,300	(27,400)	(8,900)	(131,984)	(273,652)	(1,156)
Other income			(7,600)	28,800	42,200	32,700	33,800	(13,500)	(12,900)	63,349	40,107	58,518
Expenses
Loss and LAE		148,600	214,900	123,900	(25,500)	104,800	110,800	85,700	110,900	461,890	412,161	393,800
Amortization of DAC		6,700	7,300	9,500	7,400	11,000	8,000	6,900	8,200	30,849	34,057	53,899
AGMH acquisition-related expenses								2,800	4,000		6,772	92,239
Interest expense		24,700	24,900	24,700	24,800	24,700	24,900	24,900	25,100	99,112	99,621	62,783
Other operating expenses		45,800	41,900	48,500	56,800	49,300	52,200	47,400	62,600	193,000	211,536	174,165
Income (loss) before provision for income taxes		(146,800)	1,056,000	(90,300)	215,500	(312,600)	151,600	257,000	453,300	1,034,462	549,306	116,932
Provision (benefit) for income taxes		(63,200)	294,800	(47,700)	74,900	(129,100)	(13,000)	77,900	119,800	258,842	55,599	32,110
Net income (loss)		(83,600)	761,200	(42,600)	140,600	(183,500)	164,600	179,100	333,500	775,620	493,707	84,822
Net income (loss) attributable to Assured Guaranty Ltd.		$ (83,600)	$ 761,200	$ (42,600)	$ 140,600	$ (183,500)	$ 164,600	$ 179,100	$ 333,500	$ 775,620	$ 493,707	$ 85,978
Earnings (loss) per share :
Basic (in dollars per share)		$ (0.46)	$ 4.15	$ (0.23)	$ 0.76	$ (1.00)	$ 0.89	$ 0.97	$ 1.81	$ 4.23	$ 2.68	$ 0.68
Diluted (in dollars per share)		$ (0.46)	$ 4.13	$ (0.23)	$ 0.75	$ (1.00)	$ 0.88	$ 0.95	$ 1.75	$ 4.18	$ 2.61	$ 0.66
Dividends per share	$ 0.09	$ 0.045	$ 0.045	$ 0.045	$ 0.045	$ 0.045	$ 0.045	$ 0.045	$ 0.045	$ 0.18	$ 0.18	$ 0.18