Revenue from Contracts with Customers Revenue from Contracts with Customers (Revenue, Remaining Performance Obligation) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Net income	$ 8,150	$ 9,059	$ 16,015	$ 24,988
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	(684)		$ (684)
Demand fee contracts [Member] | Mid-Stream [Member] | July - December 2018
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining Impact to Revenue	2,598		2,598
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