Fair Value Measurements (Recurring) (Details) - Recurring - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets	$ 1,231	$ 6,436
Midstream derivative instrument - Earnout derivative liability	(6,402)	(4,270)
Total net assets	(5,171)	2,166
Fair Value, Inputs, Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets	1,231	6,436
Total net assets	1,231	6,436
Fair Value, Inputs, Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Midstream derivative instrument - Earnout derivative liability	(6,402)	(4,270)
Total net assets	$ (6,402)	$ (4,270)