Equity Method Investments - Summarized financial information - White Cliffs (Details) - White Cliffs Pipeline, L.L.C. [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Summarized income statement information
Equity Method Investment, Summarized Financial Information, Revenue	$ 40,391	$ 50,184
Equity Method Investment, Summarized Financial Information, Cost of Sales	384	4,113
Equity Method Investment, Summarized Financial Information, Operating, General and Administrative Expenses	5,402	6,240
Equity Method Investment, Summarized Financial Information, Depreciation and Amortization Expense	9,592	9,256
Equity Method Investment, Summarized Financial Information, Net Income (Loss)	$ 25,014	$ 30,575