Equity Method Investments - Summarized financial information - White Cliffs (Details) - White Cliffs Pipeline, L.L.C. [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Summarized income statement information
Equity Method Investment, Summarized Financial Information, Revenue	$ 49,027	$ 42,211	$ 152,150	$ 110,018
Equity Method Investment, Summarized Financial Information, Operating, General and Administrative Expenses	8,446	4,055	25,844	16,362
Equity Method Investment, Summarized Financial Information, Depreciation and Amortization Expense	8,746	5,807	25,871	14,737
Equity Method Investment, Summarized Financial Information, Net Income (Loss)	$ 31,835	$ 32,349	$ 100,428	$ 78,919