Revenue from Contracts with Customers (Revenue, Remaining Performance Obligation) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Net income	$ 21,123	$ 3,705	$ 37,138	$ 28,693
Minimum [Member] | Mid-Stream [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining Term of Contract			4 years
Maximum [Member] | Mid-Stream [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining Term of Contract			5 years
Demand fee contracts [Member] | Mid-Stream [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining Impact to Revenue	(1,983)		$ (1,983)
Demand fee contracts [Member] | Mid-Stream [Member] | October - December 2018
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining Impact to Revenue	1,299		1,299
Demand fee contracts [Member] | Mid-Stream [Member] | 2019
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining Impact to Revenue	2,632		2,632
Demand fee contracts [Member] | Mid-Stream [Member] | 2020
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining Impact to Revenue	(3,781)		(3,781)
Demand fee contracts [Member] | Mid-Stream [Member] | 2021
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining Impact to Revenue	(3,507)		(3,507)
Demand fee contracts [Member] | Mid-Stream [Member] | 2022
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining Impact to Revenue	$ 1,374		$ 1,374