Major Customers and Concentration of Credit Risk (Details) (Revenues, Customer Concentration Risk)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Marathon Petroleum Corporation
Major Customers and Concentration of Credit Risk
Major customer percentage of total revenue	17.00%	15.00%	16.00%
ExxonMobil Corporation
Major Customers and Concentration of Credit Risk
Major customer percentage of total revenue	15.00%	13.00%	13.00%
Phillips 66
Major Customers and Concentration of Credit Risk
Major customer percentage of total revenue		11.00%