Derivative Instruments and Hedging Activities (Details 2) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Derivative Assets:
Estimated Fair Value	$ 6,844	$ 2,866
Derivative Liabilities:
Estimated Fair Value	(22,114)	(18,017)
Natural gas and crude oil contracts [Member] | Other Current Assets [Member]
Derivative Assets:
Estimated Fair Value	2,933	2,557
Natural gas and crude oil contracts [Member] | Other Non-Current Assets [Member]
Derivative Assets:
Estimated Fair Value	3,911	309
Natural gas and crude oil contracts [Member] | Other Non-Current Liabilities [Member]
Derivative Liabilities:
Estimated Fair Value	(7,714)	(8,575)
Natural gas and crude oil contracts [Member] | Derivative Liabilities [Member]
Derivative Liabilities:
Estimated Fair Value	$ (14,400)	$ (9,442)