Investments in Non-Consolidated Subsidiaries (Details 3) (White Cliffs Pipeline L L C [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
White Cliffs Pipeline L L C [Member]
Summarized income statement information
Equity Method Investment, Summarized Financial Information, Revenue	$ 30,112	$ 25,732	$ 60,785	$ 48,388
Equity Method Investment, Summarized Financial Information, Operating, General and Administrative Expenses	4,113	3,640	9,292	7,525
Equity Method Investment, Summarized Financial Information, Depreciation and Amortization Expense	4,715	4,986	9,430	9,969
Equity Method Investment, Summarized Financial Information, Net Income (Loss)	$ 21,284	$ 17,106	$ 42,063	$ 30,894