Earnings Per Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Basic earnings per share ("EPS"):
Net income (loss) attributable to Assured Guaranty Ltd.	$ (144)	$ (483)
Less: Distributed and undistributed income (loss) available to nonvested shareholders	0	0
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries	(144)	(483)
Basic shares	193.9	182.4
Basic (per share)	$ (0.74)	$ (2.65)
Diluted EPS:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries	(144)	(483)
Plus: Re-allocation of undistributed income (loss) available to nonvested shareholders of AGL and subsidiaries	0	0
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries	$ (144)	$ (483)
Basic shares	193.9	182.4
Effect of dilutive securities:
Options and restricted stock awards (in shares)	0	0
Equity units (in shares)	0	0
Diluted shares	193.9	182.4
Diluted (per share)	$ (0.74)	$ (2.65)
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	5.1	18.7