Long-Term Debt and Credit Facilities (Details 3) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		3 Months Ended			3 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Apr. 08, 2008 AGC CCS securities	Sep. 30, 2012 AGC CCS securities	Apr. 08, 2005 AGC CCS securities Trust	Sep. 30, 2012 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		$ 50
Aggregate maximum stock purchase obligation of the custodial trusts							200		200
Rate basis for income distributions					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%
Fair value gain (loss)	$ (2)	$ 2	$ (12)	$ 3