Shareholder's Equity (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012
Common stock, shares authorized	500,000,000				500,000,000
Common stock Par Value (per share)	$ 0.01				$ 0.01
Repurchases of common stock	$ 39	$ 0
Dividends, per share (in dollars per share)	$ 0.10	$ 0.090
Total Shareholders' Equity Attributable to Assured Guaranty Ltd.
Repurchases of common stock	$ 39
Common Stock
Common stock repurchases (in shares)	1,914,566