Oil and Natural Gas Operations (Unaudited) (Capitalized Costs) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Extractive Industries [Abstract]
Proved	$ 7,911,554	$ 8,069,638
Unproved	150,674	142,340
Total capitalized costs	8,062,228	8,211,978
Accumulated depreciation, depletion and amortization	3,673,569	2,663,434
Capitalized costs, net	$ 4,388,659	$ 5,548,544