Customer Deposits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		48 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2007	Dec. 31, 2010
Customer Deposits
Long-term purchase and supply agreement period, years			6
Customer deposit agreements					$ 937
Credit memoranda	7	38	14	68
Customer deposits	$ 14		$ 14			$ 27