Equity Method Investments - Summarized financial information - Glass Mountain (Details) - Glass Mountain Pipeline LLC [Member] - Investment, Name [Domain] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Summarized Financial Information, Revenue	$ 9,788	$ 8,891	$ 20,909	$ 12,744
Equity Method Investment, Summarized Financial Information, Cost of Sales	0	0	1,982	0
Equity Method Investment, Summarized Financial Information, Operating, General and Administrative Expenses	1,473	1,158	2,911	2,008
Equity Method Investment, Summarized Financial Information, Depreciation and Amortization Expense	3,932	3,770	7,976	6,118
Equity Method Investment, Summarized Financial Information, Net Income (Loss)	$ 4,381	$ 3,962	$ 8,036	$ 4,615