Long-Term Debt and Credit Facilities (Details 3) (USD $)	3 Months Ended	9 Months Ended		3 Months Ended	9 Months Ended
	Apr. 08, 2008 AGC CCS securities	Sep. 30, 2013 AGC CCS securities	Apr. 08, 2005 AGC CCS securities Trust	Sep. 30, 2013 AGM CPS securities	Sep. 30, 2013 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			$ 50,000,000.0			$ 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)	1.10%	2.50%
Auction interval (in days)				28 days
Maximum rate added to basis for income distributions, based on auction process (as a percent)				2.00%	2.00%