Employee Benefit Plans (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Weighted Average Exercise Price
Share-based compensation recorded	$ 8,100,000	$ 10,900,000	$ 7,800,000
Assured Guaranty Ltd. 2004 Long-Term Incentive Plan
Employee benefit plans
Maximum number of common shares that may be delivered	10,970,000
Number of common shares available for grant	3,384,576
Stock Options
Employee benefit plans
Vesting period of awards (in years)	3 years
Period from date of grant within which awards expire (in years)	P10Y
Summary of options issued and outstanding
Balance at the beginning of the period (in shares)	5,003,770
Options exercised (in shares)	(60,070)
Options forfeited (in shares)	(745,103)
Balance at the end of the period (in shares)	4,198,597	5,003,770
Weighted Average Exercise Price
Balance at the beginning of the period (in dollars per share)	$ 20.00
Options exercised (in dollars per share)	$ 10.34
Options forfeited (in dollars per share)	$ 20.15
Balance at the end of the period (in dollars per share)	$ 20.11	$ 20.00
Number of Exercisable Options at the end of the period (in shares)	3,808,539	4,010,822
Aggregate intrinsic value of awards outstanding	1,800,000
Weighted average remaining contractual term of awards outstanding (in years)	4.6
Aggregate intrinsic value of exercisable awards	1,200,000
Weighted average remaining contractual term of exercisable awards (in years)	4.2
Unrecognized compensation expense	1,100,000
Expected weighted-average remaining service period for recognition of unrecognized compensation cost (in years)	1.0
Weighted average assumptions used in estimation of fair value of awards granted
Dividend yield (as a percent)		0.90%	2.00%
Expected volatility (as a percent)		74.68%	66.25%
Risk free interest rate (as a percent)		2.40%	2.10%
Expected life (in years)		5	5
Forfeiture rate (as a percent)		4.50%	6.00%
Weighted Average Grant Date Fair Value (in dollars per share)		$ 11.51	$ 5.15
Total intrinsic value of awards exercised	300,000	200,000	27,000
Amount received from exercise of awards	600,000	200,000	200,000
Tax benefit related to exercise of awards	$ 0	$ 28,000	$ (17,000)