Financial Instruments (Details 2)	3 Months Ended		9 Months Ended
	Sep. 30, 2012 MBbls	Sep. 30, 2011 MBbls	Sep. 30, 2012 MBbls	Sep. 30, 2011 MBbls
Schedule of Notional Quantities for Commodity Derivative Instruments
Sales, barrels	470,000	5,189,000	1,153,000	18,000,000
Purchases, barrels	380,000	4,344,000	1,066,000	17,716,000