Long-Term Debt and Credit Facilities (Details 3) (USD $)	3 Months Ended		6 Months Ended		3 Months Ended			6 Months Ended			3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 AGC CCS securities	Jun. 30, 2011 AGC CCS securities	Apr. 08, 2008 AGC CCS securities	Jun. 30, 2012 AGC CCS securities	Jun. 30, 2011 AGC CCS securities	Apr. 08, 2005 AGC CCS securities Trust	Jun. 30, 2012 AGM CPS securities	Jun. 30, 2011 AGM CPS securities	Jun. 30, 2012 AGM CPS securities	Jun. 30, 2011 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%	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%
Fair value gain (loss)	$ 4,290,000	$ 569,000	$ (9,614,000)	$ 1,095,000	$ 1,600,000	$ 300,000		$ (6,900,000)	$ 600,000		$ 2,700,000	$ 300,000	$ (2,700,000)	$ 500,000