Oil and Natural Gas Properties (Gathering and Transportation Assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Gathering and transportation assets	$ 147,479
Less: Accumulated depreciation, depletion, amortization and impairments	(652,167)	$ (517,239)
Western Catarina Midstream [Member]
Property, Plant and Equipment [Line Items]
Gathering and transportation assets	147,479
Less: Accumulated depreciation, depletion, amortization and impairments	(1,402)
Total gathering and transportation assets	$ 146,077