Consolidation of Variable Interest Entities (Details) (USD $)	12 Months Ended				12 Months Ended		12 Months Ended
	Dec. 31, 2011 Entity	Dec. 31, 2010 Entity	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2009 Before Transition Adjustment Entity	Dec. 31, 2008 Before Transition Adjustment	Dec. 31, 2009 Transition Adjustment Entity	Dec. 31, 2009 Transition Adjustment Deconsolidated VIEs
Consolidation of variable interest entities
Total number of consolidated VIEs	33	29			4		21
Assets:
Premiums receivable, net of ceding commissions payable	$ 1,002,852,000	$ 1,167,587,000	$ 1,418,200,000	$ 15,700,000	$ 1,418,200,000		$ (19,100,000)
Deferred tax asset, net	770,943,000	1,259,125,000	1,274,200,000		1,163,000,000		111,200,000
Financial guaranty variable interest entities' assets	2,819,077,000	3,657,481,000	1,925,300,000		762,300,000		1,163,000,000
Total assets	18,091,531,000	19,841,919,000	18,034,500,000		16,779,400,000		1,255,100,000	791,900,000
Liabilities and shareholders' equity:
Unearned premium reserve	5,962,799,000	6,972,894,000	8,268,000,000		8,381,000,000		(113,000,000)
Financial guaranty variable interest entities' liabilities with recourse	2,396,945,000	3,030,908,000	2,110,900,000		762,700,000		1,348,200,000
Financial guaranty variable interest entities' liabilities without recourse	1,061,497,000	1,337,214,000	226,000,000				226,000,000
Total liabilities	13,373,095,000	16,108,465,000	14,731,700,000		13,270,500,000		1,461,200,000	788,700,000
Retained earnings	1,774,793,000	1,032,445,000	572,200,000		778,700,000		(206,500,000)
Total shareholders' equity attributable to Assured Guaranty Ltd.			3,302,800,000		3,509,300,000		(206,500,000)
Noncontrolling interest of financial guaranty variable interest entities					(400,000)		400,000
Total shareholders' equity	4,718,436,000	3,733,454,000	3,302,760,000	1,945,665,000	3,508,951,000	1,926,222,000	(206,100,000)
Total liabilities and shareholders' equity	$ 18,091,531,000	$ 19,841,919,000	$ 18,034,500,000		$ 16,779,400,000		$ 1,255,100,000
Number of VIEs deconsolidated							4