Equity Method Investments - Summarized financial information - White Cliffs (Details) - White Cliffs Pipeline, L.L.C. [Member] - Equity Method Investee, Name [Domain] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Summarized income statement information
Equity Method Investment, Summarized Financial Information, Revenue	$ 48,509	$ 34,533	$ 103,123	$ 67,807
Equity Method Investment, Summarized Financial Information, Operating, General and Administrative Expenses	9,045	5,539	17,398	12,307
Equity Method Investment, Summarized Financial Information, Depreciation and Amortization Expense	8,587	4,537	17,125	8,930
Equity Method Investment, Summarized Financial Information, Net Income (Loss)	$ 30,870	$ 24,457	$ 68,593	$ 46,570