Earnings Per Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Basic earnings per share ("EPS"):
Net income (loss) attributable to Assured Guaranty Ltd.	$ 219	$ 377	$ 75	$ (106)
Less: Distributed and undistributed income (loss) available to nonvested shareholders	0	1	0	0
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries	219	376	75	(106)
Basic shares	187.8	186.3	190.8	184.4
Basic (per share)	$ 1.17	$ 2.02	$ 0.39	$ (0.58)
Diluted EPS:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries	219	376	75	(106)
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	$ 219	$ 376	$ 75	$ (106)
Basic shares	187.8	186.3	190.8	184.4
Effect of dilutive securities:
Options and restricted stock awards (in shares)	1.0	0.7	0.9	0
Equity units (in shares)	0	0	0	0
Diluted shares	188.8	187.0	191.7	184.4
Diluted (per share)	$ 1.16	$ 2.01	$ 0.39	$ (0.58)
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	2.0	13.0	3.6	15.9