Derivative Commodity Instruments - Derivative Instruments Change in Fair Value (Details) (Derivative Commodity Instruments, USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Derivative Commodity Instruments
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	$ 103,456,000	$ 13,446,000	$ 65,801,000	$ 42,755,000
Realized losses	18,737,000	9,370,000	51,858,000	37,031,000
Unrealized gains (losses) relating to instruments held at the reporting date	(46,983,000)	23,953,000	(9,328,000)	(5,356,000)
Settlements during period	(17,929,000)	(10,565,000)	(51,050,000)	(38,226,000)
Balance at end of period	57,281,000	36,204,000	57,281,000	36,204,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset and Liability, Mark-toMarket Gain (Loss) Included in Earnings, Unrealized	7,900,000		4,500,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset and Liability, Mark-toMarket Gain Included in Earnings, Unrealized		$ 100,000		$ 100,000