Equity Method Investments - Summarized financial information - White Cliffs (Details) - White Cliffs Pipeline, L.L.C. [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Summarized income statement information
Equity Method Investment, Summarized Financial Information, Revenue	$ 49,659	$ 55,586	$ 99,843	$ 113,642
Equity Method Investment, Summarized Financial Information, Cost of Sales	4,338	2,803	8,451	3,053
Equity Method Investment, Summarized Financial Information, Operating, General and Administrative Expenses	5,886	10,125	12,126	19,727
Equity Method Investment, Summarized Financial Information, Depreciation and Amortization Expense	9,209	10,084	18,465	19,047
Equity Method Investment, Summarized Financial Information, Net Income (Loss)	$ 30,185	$ 32,575	$ 60,760	$ 71,822