Investments in Non-Consolidated Subsidiaries - Summarized financial information - White Cliffs (Details) (White Cliffs Pipeline, L.L.C. [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
White Cliffs Pipeline, L.L.C. [Member]
Summarized income statement information
Equity Method Investment, Summarized Financial Information, Revenue	$ 34,533	$ 30,112	$ 67,807	$ 60,785
Equity Method Investment, Summarized Financial Information, Operating, General and Administrative Expenses	5,539	4,113	12,307	9,292
Equity Method Investment, Summarized Financial Information, Depreciation and Amortization Expense	4,537	4,715	8,930	9,430
Equity Method Investment, Summarized Financial Information, Net Income (Loss)	$ 24,457	$ 21,284	$ 46,570	$ 42,063