Significant Accounting Policies (Major Customers) (Details Textuals 2)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Marathon Petroleum Company [Member]
Product Information [Line Items]
Revenue from major customer (percentage)	31.00%	33.00%	39.00%
Plains Marketing LP [Member]
Product Information [Line Items]
Revenue from major customer (percentage)	13.00%	15.00%	17.00%
Eighty-Eight Oil LLC [Member]
Product Information [Line Items]
Revenue from major customer (percentage)		10.00%
ConocoPhillips [Member]
Product Information [Line Items]
Revenue from major customer (percentage)	12.00%