Summary of Significant Accounting Policies (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Summary of revisions
Natural gas, NGL and oil sales	$ 331,720	$ 304,230	$ 285,353	$ 251,963	$ 231,879	$ 203,255	$ 187,523	$ 200,633	$ 1,173,266	$ 823,290	$ 751,749
Natural gas, NGL and oil sales, previously reported [Member]
Summary of revisions
Natural gas, NGL and oil sales										760,453	714,564
Revision of transportation, gathering and compression expenses [Member]
Summary of revisions
Natural gas, NGL and oil sales										$ 62,837	$ 37,185