Oil and Natural Gas (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2017	Sep. 30, 2016
Property, Plant and Equipment [Abstract]
Future discounted net cash flows discounted				10.00%
Impairment of oil and natural gas properties	$ 0	$ 0	$ 0		$ 161,600
Non-cash ceiling test write-down net of tax					$ 100,600