Derivative Instruments and Hedging Activities (Details)	Sep. 30, 2011 USD ($)	Sep. 30, 2011 Open Crude Oil Derivative Positions [Member] 2011 [Member] Fixed-Price Swaps [Member] USD ($) MMbbl	Sep. 30, 2011 Open Crude Oil Derivative Positions [Member] 2011 [Member] Collars [Member] USD ($) MMbbl	Sep. 30, 2011 Open Crude Oil Derivative Positions [Member] 2012 [Member] Fixed-Price Swaps [Member] USD ($) MMbbl	Sep. 30, 2011 Open Crude Oil Derivative Positions [Member] 2012 [Member] Collars [Member] USD ($) MMbbl	Sep. 30, 2011 Open Crude Oil Derivative Positions [Member] 2013 [Member] Fixed-Price Swaps [Member] USD ($) MMbbl	Sep. 30, 2011 Open Crude Oil Derivative Positions [Member] 2013 [Member] Collars [Member] USD ($) MMbbl	Sep. 30, 2011 Open Crude Oil Derivative Positions [Member] 2014 [Member] Fixed-Price Swaps [Member] USD ($) MMbbl	Sep. 30, 2011 Open Natural Gas Derivative Positions [Member] 2011 [Member] Fixed-Price Swaps [Member] USD ($) MMBtu GJ	Sep. 30, 2011 Open Natural Gas Derivative Positions [Member] 2011 [Member] Fixed-Price Swaps [Member] CAD	Sep. 30, 2011 Open Natural Gas Derivative Positions [Member] 2011 [Member] Collars [Member] USD ($) MMBtu GJ	Sep. 30, 2011 Open Natural Gas Derivative Positions [Member] 2011 [Member] Collars [Member] CAD	Sep. 30, 2011 Open Natural Gas Derivative Positions [Member] 2012 [Member] Fixed-Price Swaps [Member] USD ($) MMBtu GJ	Sep. 30, 2011 Open Natural Gas Derivative Positions [Member] 2012 [Member] Fixed-Price Swaps [Member] CAD	Sep. 30, 2011 Open Natural Gas Derivative Positions [Member] 2012 [Member] Collars [Member] USD ($) MMBtu GJ	Sep. 30, 2011 Open Natural Gas Derivative Positions [Member] 2012 [Member] Collars [Member] CAD	Sep. 30, 2011 Open Natural Gas Derivative Positions [Member] 2013 [Member] Fixed-Price Swaps [Member] USD ($) MMBtu	Sep. 30, 2011 Open Natural Gas Derivative Positions [Member] 2013 [Member] Collars [Member] USD ($) MMBtu	Sep. 30, 2011 Open Natural Gas Derivative Positions [Member] 2014 [Member] Fixed-Price Swaps [Member] USD ($) MMBtu
Commodity Derivative Instruments [Abstract]
Mbbls		1,405,000	7,503,000	4,110,000	12,628,000	1,972,000	2,416,000	76,000	19,965,000	19,965,000	2,300,000	2,300,000	48,349,000	48,349,000	21,960,000	21,960,000	10,095,000	6,825,000	1,295,000
GJ									12,880,000	12,880,000	920,000	920,000	43,920,000	43,920,000	7,320,000	7,320,000
Weighted Average Fixed Price		$ 74.87		$ 73.40		$ 74.29		$ 74.50	$ 5.97	6.26			$ 6.22	6.61			$ 6.74		$ 6.72
Weighted Average Floor Price			69.22		76.42		78.02				5.00	6.50			5.54	6.50		5.35
Weighted Average Ceiling price	$ 98.56		$ 96.82		$ 101.06		$ 103.06				$ 8.85	7.10			$ 7.30	7.27		$ 6.67