Quarterly Financial Information (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended				12 Months Ended
	Mar. 31, 2013		Mar. 31, 2012		Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Information (Unaudited) [Abstract]
Net earned premiums	$ 248	[1]	$ 194	[1]
Net investment income	94		98
Net realized investment gains (losses)	28		1
Net change in fair value of credit derivatives	(592)		(691)
Fair value gains (losses) on CCS	(10)		(14)
Fair value gains (losses) on FG VIEs	70		(41)
Other income	(14)		91
Loss and LAE	(48)		242
Amortization of DAC	3		5
Interest expense	21		25
Other operating expenses	60		62
Income (loss) before provision for income taxes	(212)		(696)
Provision (benefit) for income taxes	(68)		(213)
Net income (loss) attributable to Assured Guaranty Ltd.	$ (144)		$ (483)
Basic (per share)	$ (0.74)		$ (2.65)
Diluted (per share)	$ (0.74)		$ (2.65)
Dividends per share	$ 0.10		$ 0.090

[1] Excludes $18 million and $17 million for First Quarter 2013 and 2012, respectively, related to consolidated FG VIEs.