Equity Method Investments - Summarized financial information - White Cliffs (Details) - White Cliffs Pipeline, L.L.C. [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Summarized income statement information
Equity Method Investment, Summarized Financial Information, Revenue	$ 58,056	$ 54,614
Equity Method Investment, Summarized Financial Information, Operating, General and Administrative Expenses	9,852	8,353
Equity Method Investment, Summarized Financial Information, Depreciation and Amortization Expense	8,963	8,538
Equity Method Investment, Summarized Financial Information, Net Income (Loss)	$ 39,247	$ 37,723