Consolidated Statement of Shareholders' Equity (USD $) In Millions, except Share data, unless otherwise specified	Total	Total Shareholders' Equity Attributable to Assured Guaranty Ltd.	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income	Deferred Equity Compensation	Noncontrolling Interest of Financial Guaranty Consolidated Variable Interest Entities
Balance (Retroactive Application Adjustment)	$ 3,248	$ 3,248	$ 2	$ 2,585	$ 517	$ 142	$ 2	$ 0
Balance (Retrospective Application of New Deferred Acquisition Cost Guidance)	3,454	3,455	2	2,585	724	142	2	(1)
Retained earnings (Retrospective Application of New Deferred Acquisition Cost Guidance) (Retroactive Application Adjustment)	(55)
Retained earnings at Dec. 31, 2009	(55)	(55)			(55)
Balance at Dec. 31, 2009	3,509	3,510	2	2,585	779	142	2	(1)
Balance (in shares) (Retroactive Application Adjustment)			184,162,896
Balance (in shares) (Retrospective Application of New Deferred Acquisition Cost Guidance)			184,162,896
Balance (in shares) at Dec. 31, 2009			184,162,896
Increase (Decrease) in Shareholders' Equity
Net income (loss)	484	484	0	0	484	0	0	0
Net income (loss) (Retroactive Application Adjustment)	(10)
Dividends (2010 $0.18, 2011 $0.18, 2012 $0.36 per share)	(33)	(33)	0	0	(33)	0	0	0
Common stock repurchases (in shares)			(707,350)
Common stock repurchases	(10)	(10)	0	(10)	0	0	0	0
Share-based compensation and other (in shares)			289,109
Share-based compensation and other	11	11	0	11	0	0	0	0
Other comprehensive income	(30)	(30)	0	0	0	(30)	0	0
Balance at Dec. 31, 2010	3,670	3,670	2	2,586	968	112	2	0
Balance (in shares) at Dec. 31, 2010			183,744,655
Increase (Decrease) in Shareholders' Equity
Net income (loss)	773	773	0	0	773	0	0	0
Net income (loss) (Retroactive Application Adjustment)	(3)
Dividends (2010 $0.18, 2011 $0.18, 2012 $0.36 per share)	(33)	(33)	0	0	(33)	0	0	0
Common stock repurchases (in shares)			(2,000,000)
Common stock repurchases	(23)	(23)	0	(23)	0	0	0	0
Share-based compensation and other (in shares)			491,143
Share-based compensation and other	9	9	0	7	0	0	2	0
Other comprehensive income	256	256	0	0	0	256	0	0
Balance at Dec. 31, 2011	4,652	4,652	2	2,570	1,708	368	4	0
Balance (in shares) at Dec. 31, 2011			182,235,798
Increase (Decrease) in Shareholders' Equity
Net income (loss)	110	110	0	0	110	0	0	0
Dividends (2010 $0.18, 2011 $0.18, 2012 $0.36 per share)	(69)	(69)	0	0	(69)	0	0	0
Common stock issuance, net (in shares)			13,428,770
Common stock issuance, net	173	173	0	173	0	0	0	0
Common stock repurchases (in shares)			(2,066,759)
Common stock repurchases	(24)	(24)	0	(24)	0	0	0	0
Share-based compensation and other (in shares)			405,488
Share-based compensation and other	5	5	0	5	0	0	0	0
Other comprehensive income	147	147	0	0	0	147	0	0
Balance at Dec. 31, 2012	$ 4,994	$ 4,994	$ 2	$ 2,724	$ 1,749	$ 515	$ 4	$ 0
Balance (in shares) at Dec. 31, 2012			194,003,297