Derivative Instruments and Hedging Activities (Details 4) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Commodity Derivative Activity in Accumulated Other Comprehensive Income (Loss)
Unrealized gain (loss) on derivatives at beginning of period, before tax	$ 145	$ (54)
Realized amount reclassified into earnings, before tax	(41)	(6)
Net change in derivative fair value, before tax	(29)	(432)
Ineffectiveness reclassified into earnings, before tax	1	3
Unrealized gain (loss) on derivatives at end of period, before tax	76	(489)
Unrealized gain (loss) on derivatives at beginning of period, after tax	114	(19)
Realized amount reclassified into earnings, after tax	(34)	(4)
Net change in derivative fair value, after tax	1	(302)
Ineffectiveness reclassified into earnings, after tax		2
Unrealized gain (loss) on derivatives at end of period, after tax	$ 81	$ (323)