Derivative Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Derivative financial instruments on the condensed consolidated balance sheets
Current derivative financial instrument asset	$ 34,234	$ 42,803
Long-term derivative financial instrument asset	17,360	29,516
Current derivative financial instrument liability	(5,424)	(5,223)
Long-term derivative financial instrument liability	(2,999)	(4,611)
Derivative financial instruments, fair value	43,171	62,485
Commodity Contracts [Member]
Derivative financial instruments on the condensed consolidated balance sheets
Current derivative financial instrument asset	34,234	42,803
Long-term derivative financial instrument asset	17,360	29,516
Current derivative financial instrument liability	(5,424)	(5,223)
Long-term derivative financial instrument liability	(2,999)	(4,611)
Derivative financial instruments, fair value	$ 43,171	$ 62,485