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Significant Customers	12 Months Ended
Significant Customers	Dec. 31, 2017
Significant Customers [Abstract]
Significant Customers	Significant Customers We have two significant customers (Shell Oil and Sinclair), each of which has historically accounted for approximately 10% of our annual revenues. Shell Oil accounted for $1,317.9 million (9%), $1,048.2 million (10%) and $1,252.6 million (9%) for the years ended December 31, 2017, 2016 and 2015, respectively, and Sinclair accounted for $1,135.7 million (8%), $927.0 million (9%) and $1,104.9 million (8%) of our revenues for the years ended December 31, 2017, 2016 and 2015, respectively. Non-U.S. sales represented 7% of our revenues for the year ended December 31, 2017. The Canadian market represents our largest concentration of foreign sales and accounted for 4% of our revenues for the year ended December 31, 2017.