Capitalized Costs and Accumulated Depreciation Depletion and Amortization (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Natural gas and oil properties:
Properties subject to depletion	$ 5,326,056	$ 4,742,248
Unproved properties	673,609	648,143
Total	5,999,665	5,390,391
Accumulated depreciation, depletion and amortization	(1,447,300)	(1,306,378)
Net capitalized costs	$ 4,552,365	$ 4,084,013