Oil and Natural Gas (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2016	Sep. 30, 2015
Property, Plant and Equipment [Abstract]
Future discounted net cash flows discounted							10.00%
Impairment of oil and natural gas properties	$ 49,443	$ 74,300	$ 37,800	$ 329,924	$ 410,500	$ 400,600	$ 161,563	$ 1,141,053
Non-cash ceiling test write-down net of tax	$ 30,800	$ 46,300	$ 23,500	$ 205,400	$ 255,600	$ 249,400