Revenue from Contracts with Customers (Revenue, Remaining Performance Obligation) (Details) - Mid-Stream - Demand fee contracts [Member] $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining term of contract	4 years
Remaining impact to revenue	$ (3,282)
2019
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining impact to revenue	2,632
2020
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining impact to revenue	(3,781)
2021
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining impact to revenue	(3,507)
2022
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining impact to revenue	$ 1,374