Oil and Natural Gas (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2017
Property, Plant and Equipment [Abstract]
Future discounted net cash flows discounted					10.00%
Impairment of oil and natural gas properties	$ 0	$ 0	$ 74,300	$ 37,800
Non-cash ceiling test write-down net of tax			$ 46,300	$ 23,500