Long-Term Debt and Credit Facilities (Details 3) (USD $)	1 Months Ended				12 Months Ended
	Nov. 30, 2006 2006 Credit Facility	Dec. 31, 2011 2006 Credit Facility	Dec. 31, 2011 2009 Strip Coverage Facility AGM Transaction	Jul. 01, 2009 2009 Strip Coverage Facility AGM	Dec. 31, 2011 AG Re Credit Facility	Dec. 31, 2011 AGM Credit Facility	Jan. 02, 2012 Excess of loss reinsurance facility	Dec. 31, 2011 Letter of credit agreement with Bank of New York Mellon AGC	Dec. 31, 2011 Letter of credit agreement with Royal Bank of Scotland AGC
Credit facilities
Term of revolving credit facility (in years)	5 years
Minimum net worth which is required to be maintained (as a percent)				75.00%
Maximum debt-to-capital ratio which is required to be maintained (as a percent)			30.00%
Number of leveraged lease transactions due to 2008 downgrade			45
Liquidity claims on gross exposure			$ 1,000,000,000
Number of leveraged lease transactions due to 2011 downgrade			26
Liquidity claims on additional gross exposure			992,000,000
Cumulative strip par exposure terminated			593,000,000
Reduced maximum credit facility for 2014 if consolidated net worth not maintained			750,000,000
Maximum credit facility		300,000,000		1,000,000,000	200,000,000	297,500,000	435,000,000	2,900,000	2,900,000
Reduced commitment amount to trigger termination			980,500,000
Minimum percentage of aggregate consolidate net income (or loss) for July 1, 2009 through June 30, 2014 to be added to net worth requirement			25.00%
Amount to be added to net worth requirement if borrowing capacity is reduced			0
Threshold cumulative municipal losses to utilize credit facility					$ 260,000,000
Threshold cumulative municipal losses as a percent of debt service to utilize credit facility					4.50%