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Capital expenditures related to oil and natural gas producing activities (Details 2) (USD $)
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Acquisitions:      
Proved properties $ 1,817,000 [1] $ 839,000 [1] $ 3,999,000 [1]
Unproved properties 4,608,000 [1] 31,109,000 [1] 63,354,000 [1]
Exploration 26,975,000 [1] 235,906,000 [1] 41,775,000 [1]
Development 355,421,000 [1] 275,959,000 [1] 161,647,000 [1]
Total capital expenditures 388,821,000 [1] 543,813,000 [1] 270,775,000 [1]
Net change in property, plant and equipment related to future liabilities for asset retirement obligations that are excluded (10,700,000) (200,000) (1,800,000)
Proceeds from Sale of Oil and Gas Property and Equipment $ 83,600,000 $ 6,000,000 $ 12,400,000
[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 $(10.7) million, $(200,000) and $(1.8) million for the years ended December 31, 2013, 2012 and 2011, respectively.