Earnings Per Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Basic earnings per share ("EPS"):
Net income (loss) attributable to Assured Guaranty Ltd.																	$ 110		$ 773		$ 484
Less: Distributed and undistributed income (loss) available to nonvested shareholders																	0		1		0
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries																	110		772		484
Basic shares																	189.2		183.4		184.0
Basic (per share)	$ 0.38	[1]	$ 0.73	[1]	$ 2.02	[1]	$ (2.65)	[1]	$ (0.46)	[1]	$ 4.15	[1]	$ (0.23)	[1]	$ 0.76	[1]	$ 0.58	[1]	$ 4.21	[1]	$ 2.63
Diluted EPS:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries																	110		772		484
Plus: Re-allocation of undistributed income (loss) available to nonvested shareholders of AGL and subsidiaries																	0		0		0
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries																	$ 110		$ 772		$ 484
Basic shares																	189.2		183.4		184.0
Effect of dilutive securities:
Options and restricted stock awards (in shares)																	0.8		0.9		0.9
Equity units (in shares)																	0.7		1.2		4.0
Diluted shares																	190.7		185.5		188.9
Diluted (per share)	$ 0.38	[1]	$ 0.73	[1]	$ 2.01	[1]	$ (2.65)	[1]	$ (0.46)	[1]	$ 4.13	[1]	$ (0.23)	[1]	$ 0.74	[1]	$ 0.57	[1]	$ 4.16	[1]	$ 2.56
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)																	9.9		7.2		3.0

[1] Per share amounts for the quarters and the full years have each been calculated separately. Accordingly, quarterly amounts may not sum up to the annual amounts because of differences in the average common shares outstanding during each period and, with regard to diluted per share amounts only, because of the inclusion of the effect of potentially dilutive securities only in the periods in which such effect would have been dilutive.