Oil and Natural Gas Properties (Gathering and Transportation Assets) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Gathering and transportation assets	$ 148,070	$ 147,479
Less: Accumulated depreciation, depletion, amortization and impairments	(666,123)	(652,167)
Midstream
Property, Plant and Equipment [Line Items]
Less: Accumulated depreciation, depletion, amortization and impairments	(6,571)	(1,402)
Total gathering and transportation assets	141,499	146,077
Midstream | Western Catarina Midstream
Property, Plant and Equipment [Line Items]
Gathering and transportation assets	$ 148,070	$ 147,479