Oil and Natural Gas Properties (Gathering and Transportation Assets) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Gathering and transportation assets	$ 147,695	$ 147,479
Less: Accumulated depreciation, depletion, amortization and impairments	(657,259)	(652,167)
Midstream [Member]
Property, Plant and Equipment [Line Items]
Less: Accumulated depreciation, depletion, amortization and impairments	(4,840)	(1,402)
Total gathering and transportation assets	142,855	146,077
Midstream [Member] | Western Catarina Midstream [Member]
Property, Plant and Equipment [Line Items]
Gathering and transportation assets	$ 147,695	$ 147,479