Oil and Natural Gas (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Property, Plant and Equipment [Abstract]
Future discounted net cash flows discounted	10.00%
Impairment of oil and natural gas properties	$ 37,829	$ 400,593
Non-cash ceiling test write-down net of tax	$ 23,500	$ 249,400