Capital expenditures related to oil and natural gas producing activities (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Acquisitions:
Proved properties	$ 87,919,000	[1]	$ 1,817,000	[1]	$ 839,000	[1]
Unproved properties	138,683,000	[1]	4,608,000	[1]	31,109,000	[1]
Exploration	16,879,000	[1]	26,975,000	[1]	235,906,000	[1]
Development	331,400,000	[1]	355,421,000	[1]	275,959,000	[1]
Total capital expenditures	574,881,000	[1]	388,821,000	[1]	543,813,000	[1]
Net change in property, plant and equipment related to future liabilities for asset retirement obligations that are excluded	(9,000,000)		(10,700,000)		(200,000)
Proceeds from sale of oil and gas property and equipment	$ 246,600,000		$ 83,600,000		$ 6,000,000

[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 $(9.0) million, $(10.7) million and $(200,000) for the years ended December 31, 2014, 2013 and 2012, respectively.