Reporting Segments (Assets by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	$ 477,551	$ 486,685
Capital expenditures	186	4,867
Production
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	47,745	53,556
Capital expenditures		11
Midstream
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	421,122	429,523
Capital expenditures	186	4,856
Corporate
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	$ 8,684	$ 3,606