Derivative Commodity Instruments - Derivative Instruments Change in Fair Value (Details) (Derivative Commodity Instruments, USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Derivative Commodity Instruments
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	$ 104,923	$ 16,927	$ 65,801	$ 42,755
Realized losses	4,858	1,204	4,858	1,204
Unrealized gains (losses) relating to instruments held at the reporting date	12,970	7,720	67,209	(3,554)
Settlements during period	(19,295)	(12,405)	(34,412)	(26,959)
Balance at end of period	$ 103,456	$ 13,446	$ 103,456	$ 13,446