Reporting Segments (Assets by Segment) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	$ 503,422	$ 528,423
Capital expenditures	4,145	46,893
Production
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	53,240	58,623
Capital expenditures	205	441
Midstream
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	446,800	468,656
Capital expenditures	3,940	46,452
Corporate
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	$ 3,382	$ 1,144