Earnings Per Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Basic earnings per share ("EPS"):
Net income (loss) attributable to Assured Guaranty Ltd.	$ 42	$ (144)
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	42	(144)
Basic shares	182.1	193.9
Basic (per share)	$ 0.23	$ (0.74)
Diluted EPS:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries	42	(144)
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	$ 42	$ (144)
Basic shares	182.1	193.9
Effect of dilutive securities:
Options and restricted stock awards (in shares)	1.0	0
Diluted shares	183.1	193.9
Diluted (per share)	$ 0.23	$ (0.74)
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	1.5	5.1