Equity Method Investments - Summarized financial information - Glass Mountain (Details) - Glass Mountain Pipeline LLC [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Summarized Financial Information, Revenue	$ 6,898	$ 9,788	$ 15,470	$ 20,909
Equity Method Investment, Summarized Financial Information, Cost of Sales	(120)	(40)	445	1,974
Equity Method Investment, Summarized Financial Information, Operating, General and Administrative Expenses	1,618	1,513	3,463	2,920
Equity Method Investment, Summarized Financial Information, Depreciation and Amortization Expense	3,989	3,932	7,925	7,976
Equity Method Investment, Summarized Financial Information, Net Income (Loss)	$ 1,407	$ 4,381	$ 3,632	$ 8,036