Derivatives (Details)	Jun. 30, 2011	Dec. 31, 2010
Cash-flow hedges
Percentage of crude oil production covered by qualified cash flow hedges (as a percent)	3.00%	3.00%
Natural gas held in storage (in cubic feet)	11,000,000,000	28,000,000,000
Forecasted sale of natural gas from storage designated as cash-flow hedges (in cubic feet)	5,000,000,000	24,000,000,000
Crude Oil - Collars July 2011 - December 2011 (in barrels)
Cash-flow hedges
Daily volume	12,000	12,000	[1]
Average Floor	32.92	32.92	[1]
Average Cap	46.27	46.27	[1]
Natural Gas - Swaps July 2011 - March 2012 (in cubic feet)
Cash-flow hedges
Daily volume	50,000,000	50,000,000	[1]
Average Price	6.07	6.07	[1]

[1] At December 31, 2010, these contracts were outstanding with the same daily volumes and terms indicated and also covered the period from January 1, 2011 to June 30, 2011.