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Capital expenditures related to oil and natural gas producing activities (Details 3) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2015
 [1]
Dec. 31, 2014
Dec. 31, 2013
Acquisitions:      
Proved properties $ 0 $ 87,919 [2] $ 1,817 [2]
Unproved properties 0 138,683 [2] 4,608 [2]
Exploration 0 16,879 [2] 26,975 [2]
Development 0 331,400 [2] 355,421 [2]
Total capital expenditures $ 0 574,881 [2] 388,821 [2]
Net change in property, plant and equipment related to future liabilities for asset retirement obligations that are excluded   (9,000) (10,700)
Proceeds from sale of oil and gas property and equipment   $ 246,600 $ 83,600
[1] No wells were drilled in 2015.
[2] 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 of $(9.0) million and $(10.7) million for the years ended December 31, 2014 and 2013, respectively.