Fair Value Measurements (Details 2) (USD $)	9 Months Ended			3 Months Ended			9 Months Ended					3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Sep. 30, 2012 Net Profits Plan liability	Sep. 30, 2011 Net Profits Plan liability	Mar. 31, 2011 Net Profits Plan liability	Sep. 30, 2012 Net Profits Plan liability	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 3 [Member]		Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 3 [Member]		Sep. 30, 2012 Net Profits Plan [Member]	Sep. 30, 2011 Net Profits Plan [Member]	Sep. 30, 2012 Net Profits Plan [Member]	Sep. 30, 2011 Net Profits Plan [Member]
Liability measured at fair value using Level 3 inputs
Net Profits Plan Liability								$ 90,389,000	[1]	$ 107,731,000	[1]
Net Profits Plan
Discount rate to calculate liability pools currently in payout (as a percent)				12.00%			12.00%
Discount rate to calculate liability pools not currently in payout (as a percent)				15.00%			15.00%
Period of New York Mercantile Exchange Strip Pricing Used for Price Forecast	5						5
Period used for price assumptions of strip prices of liabilities (in years)							1
Percentage change in commodity prices (as a percent)							5.00%
Change in liability due to change in commodity prices by 5 percent				8,000,000			8,000,000
Percentage increase in discount rate (as a percent)							1.00%
Decrease in liability due to a 1 percent increase in discount rate				4,000,000			4,000,000
Liability measured at fair value using Level 3 inputs
Beginning balance	107,731,000	135,850,000				133,419,000	89,591,000
Net increase (decrease) in liability	(1,753,000)	(2,001,000)		5,558,000	(20,194,000)
Net settlements	(15,589,000)	(25,360,000)		(4,760,000)	(4,736,000)
Transfers in (out) of Level 3	0	0		0	0
Ending balance	90,389,000	108,489,000		90,389,000	108,489,000		90,389,000
Amount of cash payments related to divested properties												274,000	0	2,000,000	6,300,000
Amount paid for settlement of liabilities						2,600,000
Proved Oil and Gas Properties
Discount rate used for fair value of oil and gas properties (as a percent)	12.00%
Proved Oil and Gas Property, Successful Effort Method	5,197,761,000		4,378,987,000					2,057,000	[2]	139,992,000	[2]
Unproved Oil and Gas Properties [Abstract]
Unproved Oil and Gas Property, Successful Effort Method	$ 160,468,000		$ 120,966,000					$ 9,200,000	[2]	$ 15,809,000	[2]

[1] This represents a financial asset or liability that is measured at fair value on a recurring basis.

[2] This represents a non-financial asset or liability that is measured at fair value on a nonrecurring basis.