Equity Method Investments - Summarized financial information - White Cliffs (Details) (White Cliffs Pipeline, L.L.C. [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
White Cliffs Pipeline, L.L.C. [Member]
Summarized income statement information
Equity Method Investment, Summarized Financial Information, Revenue	$ 54,614	$ 33,274
Equity Method Investment, Summarized Financial Information, Operating, General and Administrative Expenses	8,353	6,768
Equity Method Investment, Summarized Financial Information, Depreciation and Amortization Expense	8,538	4,393
Equity Method Investment, Summarized Financial Information, Net Income (Loss)	$ 37,723	$ 22,113