Financial Guaranty Insurance Acquisition Costs (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Effect of Retrospective Application of New Deferred Acquisition Cost Guidance
Amortization of DAC	$ 3	$ 5
Other operating expenses	60	62
Net income (loss) attributable to Assured Guaranty Ltd.	(144)	(483)
Earnings per share:
Basic (per share)	$ (0.74)	$ (2.65)
Diluted (per share)	$ (0.74)	$ (2.65)
Deferred Aquisition Costs With Change in Accounting Principle [Roll Forward]
Balance, beginning of period	116
Balance, end of period	$ 116