Shareholder's Equity (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended																	6 Months Ended		12 Months Ended						12 Months Ended			3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011 Unit		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Jun. 30, 2012		Dec. 31, 2012		Dec. 31, 2011 Unit		Dec. 31, 2010	Aug. 31, 2011 Unit	Dec. 31, 2012 Total Shareholders' Equity Attributable to Assured Guaranty Ltd.	Dec. 31, 2011 Total Shareholders' Equity Attributable to Assured Guaranty Ltd.	Dec. 31, 2010 Total Shareholders' Equity Attributable to Assured Guaranty Ltd.	Mar. 31, 2013 Common Stock	Dec. 31, 2012 Common Stock	Dec. 31, 2011 Common Stock	Dec. 31, 2010 Common Stock	Nov. 14, 2011 Common Stock
Authorized share capital		$ 5																		$ 5
Common stock, shares authorized		500,000,000								500,000,000										500,000,000		500,000,000
Common stock Par Value (per share)		$ 0.01								$ 0.01										$ 0.01		$ 0.01
Stock Repurchase Program, Authorized Amount																													200
Common stock issuance, net (in shares)																		13,428,770	[1]											13,428,770
Price per share of stock issued (per share)																		$ 12.85	[1]
Proceeds from shares issued																		173	[1]
Proceeds from issuances of common stock																		173	[1]	173		0		0		173
Repurchases of common stock																				$ 24		$ 23		$ 10		$ 24	$ 23	$ 10
Common stock repurchases (in shares)																														2,066,759	2,000,000	707,350
Shares authorized to be repurchased (in shares)																																	5,000,000
Number of units the executives in AGC SERP elected to invest in (in units)										68,181												68,181			138,375
Dividends, per share (in dollars per share)	$ 0.10	$ 0.09	[2]	$ 0.09	[2]	$ 0.09	[2]	$ 0.09	[2]	$ 0.045	[2]	$ 0.045	[2]	$ 0.045	[2]	$ 0.045	[2]			$ 0.36	[2]	$ 0.18	[2]	$ 0.18
Increase in common stock dividend (as a percent)	11.00%																			100.00%
Dividend paid on common share (in dollars per share)																						$ 0.045		$ 0.045

[1] Relates to the settlement of forward purchase contracts. See Note 2, Business Changes, Risks, Uncertainties and Accounting Developments.

[2] 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.