Oil and Natural Gas (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2016	Jun. 30, 2015
Property, Plant and Equipment [Abstract]
Future discounted net cash flows discounted					10.00%
Impairment of oil and natural gas properties	$ 74,291	$ 37,800	$ 410,536	$ 400,600	$ 112,120	$ 811,129
Non-cash ceiling test write-down net of tax	$ 46,300	$ 23,500	$ 255,600	$ 249,400