Property and Equipment (Narrative) (Details) $ in Thousands, shares in Millions	1 Months Ended		12 Months Ended
	Aug. 31, 2017 USD ($) a shares	Jul. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Property, Plant and Equipment [Line Items]
Acquisition purchase price	$ 3,400
Mineral acres acquired (acre) | a	973
Stock issued as part of acquisition | shares	2.0
Proceeds from the sale of oil and gas properties			$ 16,979	$ 13,570	$ 138
Ward County, TX [Member]
Property, Plant and Equipment [Line Items]
Acquisition purchase price		$ 20,900
Bone Spring and Wolfcamp [Member]
Property, Plant and Equipment [Line Items]
Mineral acres acquired (acre) | a	853