Earnings Per Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Basic earnings per share ("EPS"):
Net income (loss) attributable to Assured Guaranty Ltd.	$ 384	$ 142	$ 459	$ 36
Less: Distributed and undistributed income (loss) available to nonvested shareholders	0	0	0	1
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries	384	142	459	35
Basic shares	182.9	194.0	188.2	187.6
Basic (per share)	$ 2.10	$ 0.73	$ 2.44	$ 0.19
Diluted EPS:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries	384	142	459	35
Plus: Re-allocation of undistributed income (loss) available to nonvested shareholders of AGL and subsidiaries	0	0	0	0
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries	$ 384	$ 142	$ 459	$ 35
Basic shares	182.9	194.0	188.2	187.6
Effect of dilutive securities:
Options and restricted stock awards (in shares)	1.0	0.7	0.9	0.7
Equity units (in shares)	0	0	0	1.0
Diluted shares	183.9	194.7	189.1	189.3
Diluted (per share)	$ 2.09	$ 0.73	$ 2.43	$ 0.19
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	1.9	3.9	3.0	11.9