Capital expenditures related to oil and natural gas producing activities (Details 3) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Acquisitions:
Proved properties	$ 839,000	[1]	$ 3,999,000	[1]	$ 89,733,000	[1]
Unproved properties	31,109,000	[1]	63,354,000	[1]	92,100,000	[1]
Exploration	235,906,000	[1]	41,775,000	[1]	33,226,000	[1]
Development	275,959,000	[1]	161,647,000	[1]	139,733,000	[1]
Total capital expenditures	543,813,000	[1]	270,775,000	[1]	354,792,000	[1]
Net change in property, plant and equipment related to future liabilitie for asset retirement obligations that are excluded	$ (200,000)		$ (1,800,000)		$ 11,100,000

[1] Excludes net additions/(reductions) to property, plant and equipment related to the recognition of future liabilities for asset retirement obligations associated with the plugging and abandonment of oil and natural gas wells, as discussed in Note 10, of $(200,000), $(1.8) million and $11.1 million for the years ended December 31, 2012, 2011 and 2010, respectively.