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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Summarized income statement information
Equity Method Investment, Summarized Financial Information, Revenue	$ 55,586	$ 48,509	$ 113,642	$ 103,123
Equity Method Investment, Summarized Financial Information, Cost of Sales	2,803	169	3,053	1,102
Equity Method Investment, Summarized Financial Information, Operating, General and Administrative Expenses	10,125	8,876	19,727	16,296
Equity Method Investment, Summarized Financial Information, Depreciation and Amortization Expense	10,084	8,587	19,047	17,125
Equity Method Investment, Summarized Financial Information, Net Income (Loss)	$ 32,575	$ 30,870	$ 71,822	$ 68,593