Long-Term Debt and Credit Facilities (Details 4) (USD $)	3 Months Ended								12 Months Ended			4 Months Ended	12 Months Ended					12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Apr. 30, 2008 AGC CCS securities	Dec. 31, 2011 AGC CCS securities	Dec. 31, 2010 AGC CCS securities	Dec. 31, 2009 AGC CCS securities	Apr. 08, 2008 AGC CCS securities	Apr. 08, 2005 AGC CCS securities trust	Dec. 31, 2011 AGM CPS securities D	Dec. 31, 2010 AGM CPS securities	Dec. 31, 2009 AGM CPS securities	Jun. 30, 2003 AGM CPS securities trust
Committed Capital Securities
Number of custodial trusts																	4				4
Maximum stock purchase obligation of each custodial trust																	$ 50,000,000				$ 50,000,000
Aggregate maximum stock purchase obligation of the custodial trusts																	200,000,000				200,000,000
Rate basis for income distributions												one-month LIBOR	one-Month LIBOR					one-month LIBOR
Rate added to basis for income distributions (as a percent)													2.50%			1.10%
Auction interval (in days)																		28
Maximum rate added to basis for income distributions, based on auction process (as a percent)																		2.00%
Fair value gain (loss)	$ 31,600,000	$ 2,400,000	$ 600,000	$ 500,000	$ 3,400,000	$ (5,500,000)	$ 12,600,000	$ (1,300,000)	$ 35,130,000	$ 9,195,000	$ (122,940,000)		$ 20,200,000	$ 7,100,000	$ (47,100,000)			$ 14,900,000	$ 2,100,000	$ (75,800,000)