Business Combinations (Details) (USD $)	3 Months Ended								12 Months Ended			1 Months Ended	12 Months Ended
	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	Jul. 30, 2009 AGMH	Dec. 31, 2011 AGMH	Dec. 31, 2010 AGMH	Dec. 31, 2009 AGMH	Jul. 01, 2009 AGMH	Jun. 30, 2009 AGMH Amount recorded prior to acquisition accounting adjustment	Jan. 24, 2012 MIAC jurisdiction	Dec. 31, 2011 MIAC	Sep. 30, 2011 MIAC
Acquisition
Excess of acquiree's deferred premium revenue over gross unearned premium reserves and loss and loss adjustment expense (LAE) reserve																$ 1,700,000,000
Excess of acquiree's carrying value of debt over fair value of debt																300,000,000
Purchase price:
Cash																546,000,000
Fair value of common shares issued (based upon June 30, 2009 closing price of AGL common shares)																275,900,000
Total purchase price																821,900,000
Identifiable assets acquired:
Investments																5,950,100,000
Cash																87,000,000
Premiums receivable, net of ceding commissions payable																854,100,000
Ceded unearned premium reserve																1,727,700,000
Deferred tax asset, net																888,100,000
FG VIEs' assets																1,879,400,000
Other assets																662,600,000
Total assets																12,049,000,000
Liabilities assumed:
Unearned premium reserve																7,286,400,000
Long-term debt																560,600,000
Credit derivative liabilities																920,000,000
FG VIEs' liabilities																1,878,600,000
Other liabilities																348,900,000
Total liabilities																10,994,500,000
Net assets																1,054,500,000	2,100,000,000
Bargain purchase gain resulting from the AGMH Acquisition												232,600,000			232,600,000
Components of goodwill and settlements of pre-existing relationship
Goodwill impairment associated with assumed reinsurance line of business															85,400,000
Gain on bargain purchase of AGMH												(232,600,000)			(232,600,000)
Settlement of pre-existing relationship in conjunction with the AGMH acquisition															170,500,000
Goodwill and settlements of pre-existing relationship											23,341,000				23,300,000
AGMH acquisition-related expenses
Severance costs															40,400,000
Professional services														6,800,000	32,800,000
Office consolidation															19,100,000
Total							2,800,000	4,000,000		6,772,000	92,239,000		92,300,000	6,800,000
Pro Forma Condensed Combined Results of Operations (Unaudited)
Number of U.S. jurisdictions in which the entity is licensed to provide financial guaranty insurance and reinsurance																		38
Amount of cash which the Company will pay for business acquisition																			91,000,000
Approximate amount of cash and short-term investments of the acquiree																			75,000,000
Statutory policyholders' surplus																				75,100,000
Assured Guaranty as reported
Revenue									1,819,313,000	1,313,453,000	917,159,000
Net Income Attributable to Assured Guaranty Ltd.	(83,600,000)	761,200,000	(42,600,000)	140,600,000	(183,500,000)	164,600,000	179,100,000	333,500,000	775,620,000	493,707,000	85,978,000
Net Income per Basic Share	$ (0.46)	$ 4.15	$ (0.23)	$ 0.76	$ (1.00)	$ 0.89	$ 0.97	$ 1.81	$ 4.23	$ 2.68	$ 0.68
Pro forma combined
Revenues											2,304,200,000
Net Income Attributable to Assured Guaranty Ltd.											$ 812,600,000
Net Income per Basic Share (in dollars per share)											$ 4.19